feat: generate commit messages with llm provider

[marshallhahn]

Summary

• Commit-and-push without a message generates one via the task’s LLM provider (with fallback).
• PR creation for forks uses the correct owner:branch head ref, and head-ref failures return HEAD_REF_INVALID with a clear message.

Changes

• Generate commit message from staged diff via task agent (or fallback providers); fallback to heuristic or chore: apply task changes on failure.
• Use originOwner:branch (or repo owner) for gh pr create --head so fork PRs work.
• Map “head sha can’t be blank” / “no commits between” / “head ref must be a branch” to code: HEAD_REF_INVALID and a user-friendly message.
• Refactors: explicit types in create-PR flow, shared provider fallback, test dir reorg.

Rationale

• Better commit messages when none are given
• Fork PRs need owner:branch; stable error code lets the UI show “push to origin” / “check remote” instead of raw gh output.

Testing

• Commit and push with no message
  • Generated or fallback message
• Create PR from a fork
  • PR created with correct head
• Create PR when branch isn’t on origin
  • HEAD_REF_INVALID and clear message

[vercel]

@marshallhahn is attempting to deploy a commit to the General Action Team on Vercel.

A member of the Team first needs to authorize it.

[arnestrickmann]

hi @marshallhahn, thanks for working on this.
Is this finished?