feat: commit reviewed workspace changes with explicit Git identity

[harshitethic]

Summary

Add the missing local-commit stage to PatchPilot's repository workflow.

Until now, an agent run created an isolated branch, applied edits, ran tests, and exposed a diff, but the reviewed workspace could not be turned into a Git commit through the API. This PR adds an explicit human-triggered commit boundary without pushing anything remotely.

API

POST /api/commit

{
  "workspace_id": "<workspace>",
  "message": "fix: handle expired sessions",
  "author_name": "PatchPilot",
  "author_email": "patchpilot@localhost"
}

The response includes the resulting commit SHA, current branch, normalized message, and author identity.

Behavior

• resolves the workspace using the existing containment-safe workspace lookup;
• refuses blank commit messages and clean repositories;
• stages the complete reviewed workspace with git add --all;
• verifies that staging actually produced a diff;
• creates the commit using per-command git -c user.name=... -c user.email=... values;
• never invokes a shell and does not mutate global or repository Git identity;
• returns the final SHA and branch;
• still does not push or create remote PRs.

Validation

Adds real Git integration tests that initialize a temporary repository and verify:

• a real commit is created;
• author name/email and subject are exactly what the request supplied;
• the repository is clean afterwards;
• clean workspaces are rejected;
• malformed author email is rejected by request validation.

README/API/roadmap documentation is updated to reflect the new commit stage.