ci: use code-owner-reviewed master for trusted PR workflow (#13470)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - Pull request CI uses a trusted workflow on the AWS runner fleet.
> - The caller used a fixed SHA that also needed runner-group admission.
> - A mainline pin update left CI queued because the group still allowed
older SHAs.
> - This pull request calls the trusted workflow on master, which
requires code-owner review.
> - New merged workflow versions can use the existing master
runner-group entry.

## Linked Issues or Issue Description

Related: #12968. That Dependabot PR proposes another SHA rotation. This
change keeps this first-party workflow on master instead.

**What happened?**

CI run 34975562974 stayed queued because its trusted workflow SHA was
absent from the runner-group allowlist. The fleet itself was healthy.

**Expected behavior**

New reviewed versions of the trusted workflow on master should receive
runner access without a separate SHA allowlist update.

**Steps to reproduce**

Change the caller to a new trusted workflow SHA without adding that SHA
to the restricted runner group. Its jobs remain queued. The master
reference removes that recurring synchronization step.

## What Changed

- Call `paperclipai/paperclip/.github/workflows/pr-trusted.yml@master`.
- Exclude this exact first-party workflow from Dependabot updates.
- Update the existing E2E shard workflow tests for the master caller
contract.
- Document the runner-group entry, required code-owner review, and
old-reference retention.

## Verification

- `actionlint .github/workflows/pr.yml` passed.
- `node --test scripts/__tests__/e2e-shard.test.mjs
.github/scripts/tests/cloud-runner-routing.test.mjs
.github/scripts/tests/pr-runner-rust-cache.test.mjs
.github/scripts/tests/pr-dependency-cache.test.mjs` passed: 35 tests.
- Parsed Dependabot YAML and checked the exact workflow exclusion.
- `git diff --check` passed.
- Live GitHub checks confirmed `.github/**` has code owners, CODEOWNERS
has no errors, and the active master ruleset requires code-owner review.
This covers the trusted workflow, caller, and CODEOWNERS itself.
- The approved organization setting now allows
`paperclipai/paperclip/.github/workflows/pr-trusted.yml@refs/heads/master`.
All previous references and other runner-group settings remain intact.
- Full application typecheck, tests, and build were not repeated locally
for this workflow-only change. This PR's CI and review are pending.

## Risks

New versions of the trusted workflow take effect for new callers after
merge to master. Keep code-owner review and master protection enabled.
Existing administrator pull-request bypasses remain unchanged.
Third-party action pins, runner routing, and infrastructure are
unchanged. Older callers still use their SHA pins; their allowed refs
remain in place.

## Model Used

OpenAI GPT-6 through Codex performed implementation and orchestration;
its exact runtime variant and context-window size were not exposed.
OpenAI `gpt-5.6-luna` with high reasoning inspected workflow assumptions
and applied the approved runner-group setting. Both used code and tool
access.

## 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/__tests__/e2e-shard.test.mjs

@@ -25,19 +25,15 @@ function runShard(args) {
   return result.stdout.trim().split(/\s+/).filter(Boolean);
 }
 
-function readPinnedTrustedPrWorkflow() {
+function readTrustedPrWorkflow() {
   const caller = readFileSync(prCallerWorkflow, "utf8");
-  const pin = caller.match(
-    /uses: paperclipai\/paperclip\/\.github\/workflows\/pr-trusted\.yml@([0-9a-f]{40})/,
+  assert.match(
+    caller,
+    /^\s+uses: paperclipai\/paperclip\/\.github\/workflows\/pr-trusted\.yml@master\s*$/m,
+    "pr.yml must call the trusted workflow from CODEOWNERS-protected master",
   );
-  assert.ok(pin, "pr.yml must call the trusted workflow at a full commit SHA");
-
-  const result = spawnSync("git", ["show", `${pin[1]}:${trustedPrWorkflowPath}`], {
-    cwd: repoRoot,
-    encoding: "utf8",
-  });
-  assert.equal(result.status, 0, `cannot read the pinned trusted workflow: ${result.stderr}`);
-  return result.stdout;
+  // Validate proposed workflow changes locally; CI executes the merged master version.
+  return readFileSync(trustedPrWorkflow, "utf8");
 }
 
 function readWorkflowJobs(workflow) {
@@ -161,15 +157,15 @@ test("shard arguments are validated", () => {
   }
 });
 
-test("pr.yml calls the trusted PR workflow at an immutable SHA", () => {
-  assert.ok(readPinnedTrustedPrWorkflow().length > 0);
+test("pr.yml calls the trusted PR workflow from master", () => {
+  assert.ok(readTrustedPrWorkflow().length > 0);
 });
 
 test("the trusted PR workflow keeps a stable aggregate check named e2e over the shard matrix", () => {
   // Branch protection requires a check literally named `e2e`. The shards run
   // as `e2e shard (n/3)`, so the aggregate job below is what keeps the
   // required-check contract intact — same pattern as the `verify` aggregate.
-  const workflow = readPinnedTrustedPrWorkflow();
+  const workflow = readTrustedPrWorkflow();
   const jobs = readWorkflowJobs(workflow);
 
   const aggregate = jobs.get("e2e");
@@ -293,7 +289,7 @@ test("the stacked PR scope selector runs full CI only where intended", () => {
 test("the trusted PR workflow passes the shard's spec filter to Playwright without a literal --", () => {
   // `pnpm run test:e2e -- $specs` forwards the literal separator to Playwright,
   // so the specs after it are not applied as file filters.
-  const workflow = readPinnedTrustedPrWorkflow();
+  const workflow = readTrustedPrWorkflow();
   assert.ok(
     !/pnpm run test:e2e --\s/.test(workflow),
     "pr-trusted.yml must not insert a literal `--` between `pnpm run test:e2e` and the spec filter",
@@ -306,10 +302,8 @@ test("the trusted PR workflow passes the shard's spec filter to Playwright witho
 });
 
 test("the trusted PR workflow regenerates stale stacked lockfiles", () => {
-  // Implementation PRs validate the workflow under development here. The
-  // caller remains pinned to the last merged trusted SHA until a separate
-  // activation PR advances it, so unmerged PR code never runs on trusted
-  // infrastructure.
+  // Validate the proposed workflow here. The caller executes the merged master
+  // workflow; edits to this workflow take effect after code-owner review and merge.
   const workflow = readFileSync(trustedPrWorkflow, "utf8");
   assert.match(
     workflow,