fix: prompt file needs to be read from stable branch

[shane-moore]

Issue Addressed

@claude mentions fail with cat: .github/prompts/review.md: No such file or directory on PRs branched before #844, which added the shared prompt file.

The workflow YAML runs from stable but reads the file from the PR's head branch, which may not have it.

Proposed Changes

Read the shared review prompt from stable instead of the checked out PR branch so it's always available regardless of when the PR was branched.

Additional Info

N/A

[claude-code-actions-sigp]

Review: 1 issue found

• .github/workflows/claude-mentions.yml: When handling fork PRs, origin points to the fork repo (set at checkout, line 59). git fetch origin stable will fail because the fork likely has no stable branch. Should fetch from the base repo explicitly (e.g. git fetch https://github.com/$\{{ github.repository }}.git stable). The same fix in claude-pr-review.yml is fine since that workflow always checks out the base repo.

[diegomrsantos]

Clarifying GitHub Actions behavior here (current semantics):

As of December 8, 2025, pull_request_target runs in default-branch context (for sigp/anchor, that is stable) regardless of PR base branch.

A useful mental model is:

1. Workflow source/context: which ref provides workflow definition and context values like GITHUB_REF / GITHUB_SHA.
2. Checked-out workspace: which files shell commands (cat, ls, tests) read after actions/checkout.

In this repo:

1. claude-pr-review uses pull_request_target (.github/workflows/claude-pr-review.yml, line 7), so workflow source/context is from default branch.
2. claude-mentions uses issue_comment + pull_request_review_comment (.github/workflows/claude-mentions.yml, line 6), and these events also run from workflow files on default branch.
3. But in claude-mentions, we explicitly check out PR head/fork (.github/workflows/claude-mentions.yml, lines 59-60).
4. Then cat .github/prompts/review.md (.github/workflows/claude-mentions.yml, line 84) reads from that checked-out PR workspace, not automatically from stable.

So the failure makes sense: checkout switched to a branch that didn’t contain .github/prompts/review.md yet. This PR fixes that by reading the prompt from stable explicitly.

Sources:

• GitHub Docs (events): https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows
• GitHub Changelog (announced Nov 7, 2025; effective Dec 8, 2025): https://github.blog/changelog/2025-11-07-actions-pull_request_target-and-environment-branch-protections-changes/

[diegomrsantos]

LGTM!

[jking-aus]

lgtm