fix(acpx): support Windows agent spawning (#9980)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Local Claude, Codex, Gemini, and custom ACP adapters run through the
shared embedded ACPX engine
> - That engine wrapped every local agent command in a generated Bash
script to inject environment variables and filter child stderr
> - Windows cannot directly spawn that Bash wrapper, and npm/pnpm ACP
binaries are exposed through `.cmd` shims there
> - ACPX 0.12 already supports per-session child environment variables,
so the wrapper is unnecessary
> - This pull request registers agent commands directly, injects env
through ACPX session options, captures child stderr in-process, and adds
a real Node ACP spawn smoke on Ubuntu and Windows
> - The benefit is one cross-platform spawn path with a reusable smoke
test instead of parallel shell-wrapper implementations

## Linked Issues or Issue Description

Fixes #9941.
Refs #9428 and #9771.

**What happened**

ACPX-backed local agents failed to start on Windows because Paperclip
registered a generated POSIX `.sh` wrapper as the agent command. Windows
also needs the `.cmd` npm/pnpm shim when resolving built-in ACP
binaries, and symlink creation can fail with `EPERM` for seeded
auth/skill files.

**Expected behavior**

The same ACPX engine path should spawn a real ACP agent on Windows and
Linux, forward Paperclip/runtime env without mutating `process.env`,
preserve filtered/unfiltered child stderr behavior, and fall back to
copies where Windows symlinks are unavailable.

**Steps to reproduce**

Run a local ACPX adapter on Windows with the prior wrapper path. ACPX
attempts to spawn the generated `.sh` file and the agent never
initializes.

**Deployment mode**

Local Paperclip adapters using
`packages/adapter-utils/src/acpx-engine/`.

## What Changed

- Removed generated Bash agent/env wrappers and registered local
commands directly with ACPX.
- Passed the resolved child environment through ACPX
`sessionOptions.env`, including resume retry paths.
- Added a minimal `acpx@0.12.0` package patch exposing child stderr
callbacks and allowing documented uppercase env-map keys in persisted
session options.
- Moved stderr tee/filter behavior in-process: raw stderr remains in the
per-run file while benign `nes/close` noise is omitted from live stderr.
- Preferred `.cmd` ancestor binaries on Windows and added `EPERM` copy
fallbacks for Codex auth seeding and Gemini skill materialization.
- Added a real Node ACP echo-agent spawn smoke that can run directly on
any supported platform.

## Verification

- `pnpm exec vitest run
packages/adapter-utils/src/acpx-engine/execute.test.ts
packages/adapter-utils/src/acpx-engine/spawn-smoke.test.ts` — 57 passed.
- `pnpm --filter @paperclipai/adapter-utils typecheck` — passed.
- `node --test scripts/acpx-patch-packaging.test.mjs
scripts/release-lib.test.mjs` — 10 passed.
- Full canary release dry run under Node 24.18.0 / npm 11.16.0 — passed
in an isolated scratch clone.
- `git diff --check` — passed during implementation verification.
- One-time GitHub Actions proof: [Ubuntu ACPX spawn
smoke](https://github.com/paperclipai/paperclip/actions/runs/29924348927/job/88937774579),
[Windows ACPX spawn
smoke](https://github.com/paperclipai/paperclip/actions/runs/29924348927/job/88937774558),
and [Canary Dry
Run](https://github.com/paperclipai/paperclip/actions/runs/29924348927/job/88937774497)
passed on head `f345ac69f2`; the dedicated smoke jobs are intentionally
not retained in the recurring PR workflow.

## Risks

- The ACPX stderr callback and env persistence exemption are carried as
a pnpm dependency patch until ACPX exposes/fixes those behaviors
upstream.
- Child stderr is synchronously appended to preserve ordering and
failure diagnostics; unusually high-volume agent stderr could briefly
block the Node event loop.
- The Windows-specific `.cmd` resolution and symlink `EPERM` branches
are proven by the standalone smoke test and the linked one-time
`windows-latest` run rather than a permanent CI gate.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI GPT-5.4 via Codex CLI, medium reasoning, repository/tool
execution enabled; context-window size is not exposed in this session.

## 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

---------

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

scripts/mcp-fixtures/servers/acp-echo-agent.mjs

@@ -0,0 +1,51 @@
+#!/usr/bin/env node
+import { randomUUID } from "node:crypto";
+import { createInterface } from "node:readline";
+
+function writeMessage(message) {
+  process.stdout.write(`${JSON.stringify(message)}\n`);
+}
+
+async function handleRequest(request) {
+  if (request.method === "initialize") {
+    process.stderr.write("Error handling request { method: 'nes/close' } { code: -32601 }\n");
+    process.stderr.write("paperclip-acp-echo-agent started\n");
+    return {
+      protocolVersion: 1,
+      agentCapabilities: { loadSession: false, sessionCapabilities: { close: {} } },
+      agentInfo: { name: "paperclip-acp-echo-agent", version: "1.0.0" },
+    };
+  }
+  if (request.method === "session/new") return { sessionId: randomUUID() };
+  if (request.method === "session/prompt") {
+    writeMessage({
+      jsonrpc: "2.0",
+      method: "session/update",
+      params: {
+        sessionId: request.params.sessionId,
+        update: {
+          sessionUpdate: "agent_message_chunk",
+          content: { type: "text", text: process.env.PAPERCLIP_ACPX_SPAWN_SMOKE ?? "missing" },
+        },
+      },
+    });
+    return { stopReason: "end_turn" };
+  }
+  if (request.method === "session/close" || request.method === "session/set_mode" || request.method === "session/set_config_option") return {};
+  if (request.method === "session/cancel") return null;
+  throw new Error(`Unsupported ACP method: ${request.method}`);
+}
+
+const lines = createInterface({ input: process.stdin });
+lines.on("line", async (line) => {
+  let request;
+  try {
+    request = JSON.parse(line);
+    const result = await handleRequest(request);
+    if (request.id !== undefined && result !== null) writeMessage({ jsonrpc: "2.0", id: request.id, result });
+  } catch (error) {
+    if (request?.id !== undefined) {
+      writeMessage({ jsonrpc: "2.0", id: request.id, error: { code: -32603, message: String(error?.message ?? error) } });
+    }
+  }
+});