ci(.github/workflows): bump agents-chat-action to v0.5.1 for the /api/v2 fix (#29732)

## Summary

Bump the pinned `coder/agents-chat-action` SHA so doc-check actually
runs the `/api/v2` fix.

[coder/coder#29548](https://github.com/coder/coder/pull/29548) removed
the `/api/experimental` chat mounts, and the action still called them.

[coder/agents-chat-action#43](https://github.com/coder/agents-chat-action/pull/43)
repointed the client at `/api/v2` and is merged, but this workflow pins
the action by commit SHA, so doc-check kept running the broken v0.5.0
bundle.

## Changes

- `.github/workflows/doc-check.yaml`: `coder/agents-chat-action@3e20636`
(v0.5.0) to `@9b3ac42` (v0.5.1).

`v0.5.1` is a new tag cut at the post-merge commit of
coder/agents-chat-action#43.
The pin is the commit SHA, matching the existing convention for every
pinned action in this workflow.

## Evidence

Of the eight most recent `doc-check.yaml` runs, seven failed with the
stale action:

```
Failed to look up chats by reuse labels [...]: Coder API error: Not Found
```

## This does not fix every doc-check failure

The eighth recent failure has a different cause and survives this
change:

```
doc-check-pr-29440 is dormant since 2026-09-18T20:25:12Z. The review cannot run,
because a chat cannot start its own dormant workspace
```

That is the `Check the review workspace is usable` step failing per
[CODAGT-1025](https://linear.app/codercom/issue/CODAGT-1025), which is
deliberate: the step fails loudly rather than letting a review silently
read nothing.
Individual pull requests whose review workspace has gone dormant will
keep going red until CODAGT-1025 is addressed.

## Testing

- `make lint/actions/actionlint`: pass (via pre-commit).
- Full `pre-commit-light` suite: pass.
- Verified `v0.5.1` dereferences to commit
`9b3ac42d568e9206f9cd51a97789820d6935cc8d`, which is the SHA pinned
here.

End-to-end verification needs this merged, since the workflow runs the
pinned SHA from the base branch.

<details>
<summary>Investigation notes</summary>

### Why the earlier fix did not take effect

The action fix merged at 14:39 UTC, but GitHub Actions resolves `uses:`
against the pinned SHA, not the default branch.
Nothing in coder/coder changed when the action merged, so every
subsequent run still executed the v0.5.0 bundle.

### Failure classification, eight most recent runs

| Run | Failing step | Cause |
| --- | --- | --- |
| 35742517848 | Check the review workspace is usable | CODAGT-1025,
dormant workspace |
| 35741492350 | Run doc-check via Coder Agent Chat | stale pin, reuse
label 404 |
| 35741372081 | Run doc-check via Coder Agent Chat | stale pin, reuse
label 404 |
| 35741334633 | Run doc-check via Coder Agent Chat | stale pin, reuse
label 404 |
| 35741253070 | Run doc-check via Coder Agent Chat | stale pin, reuse
label 404 |
| 35740876746 | Run doc-check via Coder Agent Chat | stale pin, reuse
label 404 |
| 35740802233 | Run doc-check via Coder Agent Chat | stale pin, reuse
label 404 |
| 35740674362 | Run doc-check via Coder Agent Chat | stale pin, reuse
label 404 |

### Note on the floating tag

The repository also carries a `v0` tag, but it still points at `v0.1.0`
and is unmaintained.
This workflow pins exact SHAs, so that tag is not used here and was left
alone.

</details>

Refs
[DOCS-987](https://linear.app/codercom/issue/DOCS-987/bump-the-pinned-agents-chat-action-sha-in-doc-checkyaml-to-pick-up-the).

> [!NOTE]
> Coder Agents generated this pull request on Nick Vigilante's behalf.

.github/workflows/doc-check.yaml

@@ -349,7 +349,7 @@ jobs:
 
       - name: Run doc-check via Coder Agent Chat
         if: steps.check-secrets.outputs.skip != 'true' && steps.check-paths.outputs.skip != 'true'
-        uses: coder/agents-chat-action@3e20636f47ed88d1486b767ade8cb1e13a1effa7 # v0.5.0
+        uses: coder/agents-chat-action@9b3ac42d568e9206f9cd51a97789820d6935cc8d # v0.5.1
         with:
           coder-url: ${{ secrets.DOC_CHECK_CODER_URL }}
           coder-token: ${{ secrets.DOC_CHECK_CODER_SESSION_TOKEN }}