ci: activate stacked lockfile regeneration (#12464)

## Thinking Path

> - Paperclip uses trusted GitHub Actions workflows to verify each pull
request.
> - The caller pins the reusable workflow to an immutable merged commit.
> - Native stacked pull requests can inherit a stale lockfile from a
parent layer.
> - Pull request #12461 added safe merge-tree lockfile regeneration to
the trusted workflow.
> - The caller must now select that merged workflow version.
> - This pull request advances the immutable pin to the merge commit
from #12461.
> - The benefit is reliable verification for native stacked pull
requests.

## Linked Issues or Issue Description

Refs #12461

## What Changed

- Pin the trusted pull request workflow to merged commit
`1da6b37fc56dacf5e7ffbd31da35756a1cba41f8`.
- Activate stale lockfile regeneration for native stacked pull requests.

## Verification

- `actionlint .github/workflows/pr.yml`
- `node --test scripts/__tests__/e2e-shard.test.mjs`

## Risks

- Low risk. This changes only the immutable reusable-workflow pin.
- The target commit is merged into `master` and passed all required
checks.
- The target workflow adds one lockfile-only install to the policy job.

> 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 Codex with model `gpt-5`, reasoning, tool use, and code
execution.

## 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
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

.github/workflows/pr.yml

@@ -10,4 +10,4 @@ permissions:
 
 jobs:
   ci:
-    uses: paperclipai/paperclip/.github/workflows/pr-trusted.yml@f9c32513b2fe62586c60fa0b8863ebd21e5c7603
+    uses: paperclipai/paperclip/.github/workflows/pr-trusted.yml@1da6b37fc56dacf5e7ffbd31da35756a1cba41f8