feat: Add git worktree isolation for concurrent issue handling

[roger704]

Summary

This PR adds git worktree isolation to allow concurrent handling of multiple GitHub issues without workspace conflicts.

Problem

When handling multiple GitHub issues concurrently, they all share the same workspace directory (/workspace/REPO/). This causes conflicts:

1. Issue #29 creates a branch and makes code changes
2. Issue #26 webhook arrives, triggers git cleanup/reset
3. Issue #29's uncommitted work is destroyed

This is especially problematic for agents that take time to complete tasks.

Solution

Each issue/PR now gets its own isolated git worktree:

/workspace/REPO/           # Base repo (shared git history)
/workspace/REPO-issue-29/  # Worktree for issue #29
/workspace/REPO-issue-26/  # Worktree for issue #26

Features Added

Feature	Description
Worktree per issue	getOrCreateWorktree() creates isolated worktrees
Retry logic	3 attempts with exponential backoff for transient failures
Base repo check	Detects missing repo after container restart and clones first
Always update cwd	Session always uses correct worktree path, not just on first request
Auto cleanup	Worktrees cleaned when PR is merged or issue is closed
Stale cleanup	Worktrees older than 7 days are automatically removed

Benefits

• Concurrent work - Multiple issues can be worked on simultaneously
• Disk efficient - Worktrees share git history with base repo
• Container restarts - Handles gracefully by detecting missing repos
• Self-cleaning - Prevents disk exhaustion from accumulated worktrees

Changes

• Modified getOrCreateCodebaseForRepo() to accept optional issueNumber parameter
• Added getOrCreateWorktree() with retry logic
• Added cleanupWorktree() and cleanupStaleWorktrees()
• Handle closed events to trigger cleanup
• Always update conversation cwd to current worktree path

Testing

Tested in production with concurrent issue handling:

• Multiple issues processed simultaneously without conflicts
• Worktrees created and cleaned up correctly
• Container restarts handled gracefully