feat(planning): add "auto-engineer" workflow for GitHub issues

[groovecoder]

Add an "auto-engineer" workflow: For each target repo, it looks for any existing open "auto-engineer"ed Pull Requests.

• If none exist:
  1. Scans the open issues of the repo
  2. Determines the best next issue to try to work on
  3. Runs a claude step which creates a plan file for the issue
  4. Submits a pull request against the repo with the plan file in .blender/plans/
• If one does exist:
  • Has it been approved?
    • If the PR is a "plan PR" - i.e., a CODEOWNER has approved the plan file in .blender/plans/:
      • Spin up a claude code process to implement the plan and add a commit to the PR for it
    • If the PR already includes implementation commits too:
      • Spin up a claude code process that does a final self-review to check if/how the implementation has drifted from the original plan.
      • Add a summary self-review comment to the PR.
  • If it has review comments after the most recent commit
    • Spin up a claude code instance to address the review comments and add a new commit to the PR

The goal is for BLEnder to automatically work on GitHub issues and collaborate with the repo CODEOWNERS via the typical GitHub Pull Request development cycle. BLEnder should only ever have a single "auto-engineer" PR open so it doesn't overwhelm the CODEOWNERs.

[groovecoder]

Added in #56