feat: improve issue-to-PR creation flow #234
Description
Summary
Streamline the flow from an issue to creating a pull request. Currently the ClaimFlow handles branch creation and forking, but there's no simple one-click path to create a PR once code is pushed.
Goals
- When a branch exists and has commits, offer a direct "Create PR" action from the work item page
- Pre-fill PR title and body from the linked issue
- Link the PR back to the issue automatically (e.g. "Closes #N")
- Smooth integration with the existing ClaimFlow
Context
GitHub's web UI shows a "Create pull request" button on issues when a branch is linked. Flumen should offer the same or better experience.