feat(actions): add open-pr-if-needed composite

[alex-nsheaps]

Summary

Adds a new reusable composite action .github/actions/open-pr-if-needed that idempotently opens a pull request between two branches that already exist on the remote. Extracted from the pattern in nsheaps/.github/.github/workflows/sync-repo-settings.yaml (the "try direct push, fall back to PR" idiom) per Nate's review on PR #48.

Why

PR #48 inlines a "create PR if not already open" bash block in sync-main-to-edge.yaml. Nate's review noted this pattern should live in a shared composite action so the idempotency check + workflow-run-link convention are applied consistently across nsheaps workflows.

What's in this PR

• .github/actions/open-pr-if-needed/action.yml — the composite
  • Inputs: title, body, base, head, repo (defaults to github.repository), token, labels, reviewers
  • Outputs: pr-number, pr-url, result (existing | opened | nodiff)
  • Behaviour:
    1. Skips if an open PR for head -> base already exists.
    2. Appends Workflow run: <server_url>/<repo>/actions/runs/<run_id> to the body (matches the nsheaps self-link convention used in check.yaml).
    3. Tolerates the "no diff between head and base" case as a no-op (result=nodiff) instead of failing the step.
• .github/actions/open-pr-if-needed/README.md — usage docs + example.

Scope

Pre-condition: both head and base must already exist on the remote. This action does NOT push commits — for the "commit local changes then open PR" pattern, peter-evans/create-pull-request remains the right tool (and is still what sync-repo-settings.yaml uses).

Follow-up

Once this lands, PR #48 will be updated to call nsheaps/github-actions/.github/actions/open-pr-if-needed@main instead of its inline bash block.

Test plan

• YAML parses (validated locally — python3 -c 'yaml.safe_load(open(...))')
• CI green on this branch
• Henry review
• After merge: PR feat(sync-main-to-edge): open a PR when direct push can't fast-forward #48 follow-up commit swaps inline block for uses: ./.github/actions/open-pr-if-needed, then exercises the path by re-running the sync workflow against a diverged edge branch.