fix(ci): route trusted stacked PRs to AWS (#12455)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - Pull request checks protect the application and its contributors.
> - The trusted CI workflow sends approved contributors to isolated AWS
runners.
> - The workflow currently limits AWS routing to pull requests that
target master.
> - Stacked pull requests target another branch and stay in the GitHub
queue.
> - This pull request keeps the identity checks and accepts a live
nonempty base branch in the Paperclip repository.
> - The benefit is that trusted stacked pull requests can use the AWS
fleet automatically.

## Linked Issues or Issue Description

**What existing behavior does this improve?**

The trusted pull request runner gate currently sends only master-based
pull requests to AWS.

**Subsystem affected**

GitHub Actions pull request routing.

**Current behavior**

A trusted contributor can pass all numeric identity checks. The gate
still selects GitHub-hosted runners when the pull request targets
another branch in a stack.

**Proposed behavior**

The gate accepts any nonempty base branch in the Paperclip base
repository. It still verifies the live base ref, base SHA, head SHA,
merge SHA, author ID, sender ID, and triggering actor ID.

**Reason and benefit**

Large pull request stacks currently add all heavy jobs to the limited
GitHub-hosted queue. This change lets approved contributors use the
isolated AWS fleet for those jobs.

**Breaking changes**

Trusted pull requests that target a non-master branch now use AWS
instead of GitHub-hosted runners. Untrusted pull requests keep the
current GitHub-hosted route.

**Additional context**

Pull request #12339 is the master-based first item in a stack. Its child
pull requests show this queue pattern.

## What Changed

- Replace the master-only route check with a nonempty base-ref check.
Keep the existing base-repository and live-state checks.

## Verification

- Ran actionlint on .github/workflows/pr-trusted.yml.
- Ran the external routing test suite. It passed the trusted
stacked-base case and all fail-closed identity cases.

## Risks

The AWS trust boundary now includes code from a non-master base branch
when the current pull request author, sender, and triggering actor are
all approved numeric GitHub IDs. An approved account can already submit
arbitrary head code. The runner remains isolated and has no production
access or repository secrets.

> For core feature work, check 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 on GPT-5. The exact deployment ID and context-window size
are not exposed. The model used reasoning, tool use, GitHub API access,
and local 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
- [x] My branch name describes the change and contains no internal
Paperclip ticket id
- [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

.github/workflows/pr-trusted.yml

@@ -77,7 +77,7 @@ jobs:
           [[ "$EVENT_REPOSITORY" == 'paperclipai/paperclip' ]] || fail_closed 'unexpected repository'
           [[ "$EVENT_REPOSITORY_ID" == '1170821064' ]] || fail_closed 'unexpected repository ID'
           [[ "$EVENT_BASE_REPOSITORY_ID" == '1170821064' ]] || fail_closed 'unexpected base repository ID'
-          [[ "$EVENT_BASE_REF" == 'master' ]] || fail_closed 'unexpected base branch'
+          [[ -n "$EVENT_BASE_REF" ]] || fail_closed 'missing base branch'
           [[ "$EVENT_ACTION" =~ ^(opened|reopened|synchronize)$ ]] || fail_closed 'unsupported pull_request action'
 
           jq -e '