feat(git): workspace turn checkpointing via git shadow refs for safe rollback

[cagdasyurekli]

Resolves #614

Summary

Implements automatic workspace turn checkpointing via lightweight git shadow refs (refs/openworker/checkpoints/<session_id>/<turn_index>) and introduces the revert_turn tool and engine method for safe workspace rollback without polluting git history or HEAD.

Key Changes

1. Lightweight Shadow Ref Mechanism (coworker/tools/git.py):
   • create_checkpoint(workspace, session_id, turn_index): Uses an isolated temporary index file (GIT_INDEX_FILE=<tmp>) to stage all tracked, modified, and untracked changes (git add -A), write the tree (git write-tree), create a commit (git commit-tree), and point a shadow ref (refs/openworker/checkpoints/<session_id>/<turn_index>) to it. Leaves the repository's real index, HEAD, and working branches untouched.
   • list_checkpoints(workspace, session_id): Enumerates available shadow checkpoint refs.
   • restore_checkpoint(workspace, session_id, turn_index): Restores workspace files to the checkpointed snapshot via git checkout <ref> -- ., and safely prunes newly created untracked files while preserving .git and .gitignored files.
   • revert_turn tool: Registered under git_tools, exposing an interactive or programmatically invokable rollback tool.
2. TurnEngine Integration (coworker/engine.py):
   • Automatically snapshots the workspace before the first local write tool (write_local_file, replace_file_content, run_shell, apply_patch, etc.) executes in a given turn.
   • Provides engine.revert_turn(turn_index) to rollback the working tree.
3. Session & Catalog Wiring (coworker/agent.py, coworker/agents/base.py, coworker/catalog.py):
   • Passes session_id to AgentContext and git_tools.
4. Risk Classification (coworker/risk.py):
   • Classifies revert_turn as RiskClass.WRITE_LOCAL requiring explicit confirmation/approval where applicable.
5. Comprehensive Tests (tests/test_git_checkpoints.py):
   • Added unit tests covering checkpoint creation, listing, restoration of modified/deleted files, removal of newly created files, preservation of .gitignored files, and engine integration during agent turns.