Pin plugin Runner cleanup to its physical task (#19300)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - Plugin Runner owners manage a physical task through an environment
lease.
> - The existing replacement path can assign a new physical task to that
same lease.
> - A delayed old owner can then stop the replacement or mark it
stopped.
> - Cleanup must keep the original physical task identity across RPC and
database writes.
> - This pull request pins that identity and keeps cleanup available to
the new owner.

## Linked Issues or Issue Description

Refs #19134 and #19157. This fix is based on master and does not enable
repeated stopped-source recovery. A search found no duplicate fix.

**What happened?**

The maintained `createOwnedSandbox` replacement path changes
`providerLeaseId` on one lease. An old task owner uses that mutable
lease for a later Stop. A stopped-state response already in flight can
also update the new task's flags.

**Expected behavior**

The old owner must address its original task. Its late cleanup writes
must not alter the replacement. The replacement's own owner must still
stop and complete it.

**Steps to reproduce**

1. Create a plugin task owner for a stopped task.
2. Replace that task through the existing describe/startup path on the
same lease.
3. Resume an old checkpoint or a held old status response.
4. Observe a Stop for the replacement or an incorrect stopped flag
before this fix.

## What Changed

- Require each task owner to keep the physical ID returned by its
description. Reject a rebind to a different task.
- Carry the ID through the existing internal task context after startup
ends. Reject a changed physical task before RPC dispatch.
- Match the physical task in stopped-state, completion and
credential-handoff writes.
- Hold the exact lease row through the retained-session database update
and local checkpoint save.
- Cover the maintained replacement path with real PostgreSQL tests.
Preserve positive cleanup for the replacement owner. Document this
boundary.

## Verification

- Both replacement regressions failed on unmodified master `10a7d039c8`:
delayed old-owner Stop targeted the replacement, and a held old status
response marked it stopped.
- Six focused files passed 160 tests: task authority, task runtime, warm
tasks, execution target, credential writeback and task owner.
- Four selected executor factory cases passed. They cover task-only
providers and the physical task binding after the startup context ends.
- Runner TypeScript, plugin SDK and adapter-utils builds passed. Server
TypeScript check passed. Diff and added-line secret scans passed.
- All 15 setup-probe cases passed with the actual Runner and fake
provider binaries. The first local attempt lacked cargo on PATH. A
sandboxed native attempt timed out in seven process cases; the unchanged
run with loopback permission passed all 15. A separate initial DB
integration attempt was denied a loopback listener; its
permission-correct run passed.
- Review found three missed constructors in the existing SDK/Core
lifecycle fixture. They now use the actual describe result. All four
cases in that real PostgreSQL and worker RPC file passed.
- The first GitHub attempt failed in the unchanged GitHub recovery
pagination case in `chat-channels.integration.test.ts`: its zero-request
assertion observed a configuration GET. That exact case passed locally
without code or assertion changes. The one failed-job-only retry passed;
the initial failure log is retained.
- [GitHub workflow
42082587576](https://github.com/paperclipai/paperclip/actions/runs/42082587576)
passed on its second attempt. All CI checks completed, and Snyk passed.
Standard Greptile scored exact head `fb914d98b99e` 5/5 with no
unresolved threads. The branch is mergeable and ready for review.

## Risks

The task context change is internal. It adds no plugin wire field,
schema or Runner mode. A stale owner now gets an error and cannot clean
up a replacement. A new owner can still clean up that task. The retained
checkpoint write holds one existing lease row during a bounded local
file write.

Repeated stopped-source incarnation admission, runtime pins and
deployments are outside this change.

## Model Used

OpenAI GPT-6.1 Sol (`gpt-6.1-sol`), high reasoning. Used repository
tools, TypeScript and real PostgreSQL tests.

## 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 described the issue in-PR using the bug template
- [x] I have not referenced internal or instance-local Paperclip issues
or links
- [x] My branch name describes the change
- [x] I have run the focused tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] I have updated relevant documentation
- [x] I have considered and documented risks
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no unresolved findings
- [x] I will address reviewer comments before requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

doc/plugins/PLUGIN_ENVIRONMENT_TASKS.md

@@ -157,6 +157,11 @@ An accepted stop does not prove termination. Set `executionStopped: true`
 only after the Runner and all descendants have stopped. A terminal phase alone
 does not permit Core to release execution authority.
 
+Core binds each Runner owner to the physical task ID returned by its description.
+An old owner cannot follow a changed lease to stop or complete its replacement.
+Late stopped-state, retained-session and credential-handoff writes must still
+match that same physical task. A new owner can clean up the replacement.
+
 Core fences new submissions when cleanup starts. It stops the saved task,
 checks termination, saves credential results, and completes the task before it
 releases or destroys the environment lease. An uncertain result leaves a saved