feat: add upstream-sync composite action

[jack-nsheaps]

Summary

• Extracts all upstream sync logic from nsheaps/claude-code-py's raw workflow shell into a reusable composite action
• Action lives at .github/actions/upstream-sync/action.yml
• Inputs: upstream-repo (required), token (required), dry-run (optional, default false)
• Outputs: synced-branches, conflict-branches, backed-up-branches, new-branches, skipped-branches
• Handles fast-forwards, divergence (backup + force-reset), and new branches — identical logic to the original workflow
• Writes a job summary step with emoji-annotated results

Test plan

• Verify action syntax is valid YAML
• Test by calling from nsheaps/claude-code-py upstream-sync workflow (see companion PR)
• Confirm dry-run mode works (no pushes)
• Confirm fast-forward, divergence, and new-branch cases all surface correctly in step summary

Co-Authored-By: Jack Oat (https://github.com/nsheaps/.ai-agent-jack) <jack-nsheaps[bot]@users.noreply.github.com>