chore(T-36): add PR rebase workflow with AI conflict resolution

[raschmitt]

Task/Issue

T-36 · PR rebase workflow

What?

• Create .agents/skills/pr-rebase/SKILL.md with instructions for fetching PR branch, rebasing onto main, resolving conflicts with AI, and force-pushing with lease
• Create .github/workflows/pr-rebase.yml — thin workflow_dispatch wrapper accepting a pr_number input and calling the pr-rebase skill via opencode/big-pickle
• Handle no-conflict case: skip AI resolution and push directly
• Handle failed AI resolution: post a comment on the PR listing unresolvable files

Why?

• Developers need a way to rebase stale PR branches without manual intervention
• AI conflict resolution reduces friction by intelligently merging both sides of a conflict
• Protected config files (e.g. .github/workflows/) should always prefer the main branch version during rebase conflicts

[raschmitt]

🤖 This PR was automatically generated by the auto-implement workflow using OpenCode CLI with the big-pickle model.

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[raschmitt]

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