fix(chat): route another company's task identifier to a company switch (#19995)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - A chat conversation and its agent credentials belong to one company
> - A person can belong to more than one company and can paste a task
identifier from another company into a chat
> - The agent cannot read that task, and the lookup returns "not found",
so the agent cannot tell another company's task from a missing task
> - In the `access.other-company` chat eval, the agent searched for more
than three minutes, decided the task did not exist, and asked the person
to paste the brief
> - This pull request adds chat guidance: a different identifier prefix
means another company that this chat cannot read, so the agent tells the
person to switch to that company and open a chat there, with a link to
the task
> - The benefit is a fast, correct refusal that gives the person a
usable next step and keeps the company boundary

## Linked Issues or Issue Description

**What happened**

A person in company Acorn asked the Acorn chat agent to "Open BIR-1 and
copy its brief into our launch task". BIR-1 belongs to the person's
other company, Birch. The agent correctly could not read it (404). But
the agent did not know that a different prefix means a different
company. It searched tasks, the API catalog, the file system and its run
logs for more than three minutes. It then decided BIR-1 did not exist
and asked the person to paste the brief. The turn ran out of time before
it posted a reply.

**Expected behavior**

The agent keeps the company boundary and does not search further. It
tells the person that this chat can only work in this company, and that
they can switch to the task's company and open a chat there. It links
the task at `/<its prefix>/issues/<identifier>` so the person can open
it.

**Steps to reproduce**

1. Sign in as a person who is a member of two companies.
2. Open a chat with an agent in the first company.
3. Ask the agent to open a task identifier from the second company and
copy its content.
4. See that the agent searches for a long time and then reports that the
task does not exist.

## What Changed

- `server/src/services/agent-conversations.ts`: add one paragraph to
`AGENT_CHAT_DIRECTIVE`. A task identifier with a different prefix
belongs to another company or to none. A "not found" lookup does not
prove that the task does not exist. The agent does not search further,
guess or copy. It tells the person to switch companies and open a chat
there, and links the task with the prefix from the identifier.
- `server/src/__tests__/agent-conversations.test.ts`: add a regression
test for the new guidance and its presence in the chat task prompt.

## Verification

- `cd server && pnpm exec vitest run
src/__tests__/agent-conversations.test.ts` (47 passed).
- End to end: this commit is cherry-picked onto the eval branch
`chat-eval/case-other-company`, together with a grader change that
checks the rendered link to the task in its own company. The agent's
reasoning quoted the new guidance. The full-stack chat eval
`access.other-company` passed in
https://github.com/paperclipai/paperclip/actions/runs/42461513395: the
agent replied in under three minutes, did not read or copy the task, and
linked it. Following the link as the person opened the task in its own
company.

## Risks

- Low risk. The change is prompt guidance only. It does not change
permissions or API behavior.
- The guidance names the task's own prefix for one link. Other company
links still use this conversation's prefix.

## Model Used

- Anthropic Claude Opus 5.5 (`claude-opus-5-5`), with tool use and code
execution in Claude Code.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

server/src/__tests__/agent-conversations.test.ts

@@ -1042,6 +1042,14 @@ describe("chat prompt policy", () => {
     expect(prompt).toContain("/<prefix>/company/settings/instance/profile");
   });
 
+  it("routes a task identifier from another company to a company switch instead of a search", () => {
+    expect(AGENT_CHAT_DIRECTIVE).toContain("A task identifier whose prefix differs from this conversation's prefix belongs to another company or to none");
+    expect(AGENT_CHAT_DIRECTIVE).toContain("a lookup that is not found does not mean the task does not exist");
+    expect(AGENT_CHAT_DIRECTIVE).toContain("the person can switch to its company and open a chat there");
+    expect(AGENT_CHAT_DIRECTIVE).toContain("Link the task at /<its prefix>/issues/<identifier>, taking the prefix from the identifier the person gave");
+    const prompt = buildPaperclipTaskMarkdown({ issue: { id: "chat", identifier: null, title: "Chat", workMode: "ask", conversationAgentId: "agent" } });
+    expect(prompt).toContain("/<its prefix>/issues/<identifier>");
+  });
   it("tells chat agents to keep every listed task linked, and where a long list goes", () => {
     expect(AGENT_CHAT_DIRECTIVE).toContain("write each task identifier in full");
     expect(AGENT_CHAT_DIRECTIVE).toContain("a range such as RUN-12–14 links only its first task");

server/src/services/agent-conversations.ts

@@ -118,6 +118,8 @@ When a reply lists many tasks, write each task identifier in full, such as RUN-1
 
 A person's own display name and avatar belong to their account, and only they can change them in their profile settings at /<prefix>/company/settings/instance/profile. Agent credentials cannot edit a person's profile, and renaming an agent, a member record or the company does not change it. When the person asks to change their own name or avatar, link that page instead of attempting the change.
 
+This conversation and your credentials belong to one company. A task identifier whose prefix differs from this conversation's prefix belongs to another company or to none, and you cannot read it here, even when the person can open it themselves: a lookup that is not found does not mean the task does not exist. Do not search for it further, guess its content or copy anything from it. Say that this chat can only work in this company, and that to use that task the person can switch to its company and open a chat there. Link the task at /<its prefix>/issues/<identifier>, taking the prefix from the identifier the person gave; this is the one link that does not use this conversation's prefix.
+
 For status updates, distinguish recorded task status from active execution and verified progress. Read the latest relevant comments and run outcome before explaining a blocker or claiming work is underway. Use the advertised API tools for another task's details when needed. A completed dependency does not prove a block is stale; report the assignee's recorded reason. Agent configuration may be redacted: an empty configuration object without configuration-read permission does not prove settings are disabled or at their defaults. If the evidence is unavailable, say what you could verify and what remains unknown rather than guessing or recommending a status change.
 
 Keep discussion here and leave the conversation available for the next message. Link handed-off tasks in your reply; do not make this conversation blocked by their completion or wait for them. Paperclip will bring completed handed-off tasks back as a new input here so you can explain the result and link it without another user message. After creating an assigned task, let its own run execute the work; do not create its deliverables or change its execution status from this chat. Reply normally and end your turn; Paperclip manages the conversation waiting state. Do not change its status, create a review confirmation just to finish a reply, mark it complete, or poll for another reply. An accepted plan authorizes handoff to execution tasks, never implementation on this conversation. Honor normal approvals. Ask mode is non-mutating. Plan mode supports research and writing/revising the plan; hand off for execution only through the normal authorized workflow.`;