Enrich commits with pull request and issue metadata

[tabkram]

Why

A pull request title and body are written for humans. A commit subject is written for git log. When a repository squash-merges, the PR number is already sitting in the commit subject as (#123) and we throw it away.

github/copilot-release-notes is built entirely on this — it walks merged PRs rather than commits — and it is the main reason its output reads well with zero configuration.

Scope

• Parse PR and issue references out of commit subjects and bodies ((#123), Closes #45, Fixes #45).
• When a GitHub remote is present and credentials are available, resolve those references to title, body, labels, and author.
• Feed the resolved metadata into the prompt as an additional labelled, untrusted block.
• Degrade silently. No remote, no credentials, no network, a non-GitHub host, or a rate limit must all fall back to commit-only behaviour without failing the run. This tool must keep working on a private repo with no token.

Prefer the user's existing gh CLI when present, falling back to a plain token from the environment. Do not add a hard dependency on either.

Acceptance criteria

• Reference parsing covers (#N), Closes/Fixes/Resolves #N, and full URLs
• Resolution is batched, not one request per commit
• Every failure mode above falls back to commit-only, with one clear note in the run report
• Works with gh auth or GITHUB_TOKEN; neither is required
• Resolved PR text is treated as untrusted input under the scope guard
• Eval fixtures include one squash-merge repo showing the improvement

Files

src/git.ts, new src/forge.ts (or similar), src/prompts/builder.ts, src/config.ts, docs/configuration.md

Notes

Keep the forge layer behind an interface. GitLab and Bitbucket are the obvious follow-ups and should not require reworking this.