fix(monitor): handle deleted worktrees gracefully

[gregpriday]

Summary

Fix for WorktreeMonitor crashing the display when monitoring deleted worktrees. When worktrees are deleted externally (e.g., during automated PR merges), the monitor now gracefully stops polling and emits a removal event instead of flooding stderr with errors.

Closes #233

Changes Made

• Add WorktreeRemovedError class for distinguishing deleted worktrees from other git errors
• Add directory existence check in getWorktreeChangesWithStats() before calling simpleGit()
• Handle WorktreeRemovedError in WorktreeMonitor.updateGitStatus() to stop polling and emit sys:worktree:remove event
• Add comprehensive tests for deleted worktree scenarios

Implementation Notes

Context & rationale:

• When worktrees are deleted externally (e.g., during automated PR merges), WorktreeMonitor continued polling them, causing simple-git to throw errors that flooded stderr and destroyed the terminal display
• Added proactive directory existence check before calling simpleGit(cwd) to catch ENOENT early
• Created dedicated WorktreeRemovedError class for clear error differentiation
• Monitor now gracefully stops and emits removal event so UI can clean up

Implementation details:

• Added WorktreeRemovedError class in src/utils/errorTypes.ts - extends GitError with path context
• Added fs.access() check in getWorktreeChangesWithStats() before simpleGit() call
• Added error handling in WorktreeMonitor.updateGitStatus() that detects WorktreeRemovedError, emits sys:worktree:remove, and calls stop()
• Added comprehensive tests for both the git utility and WorktreeMonitor

Breaking Changes

None - internal resilience improvement