Git integration: branch picker + worktrees

[juliusmarminge]

Closes #7 (Does not implement multiple threads per worktree)

Summary

Users can now right-click on "+ New thread" to create threads from specific git branches, with optional worktree creation for isolated agent work.

Features

• Branch context menu showing local branches sorted by current/default/alphabetical
• Option to create git worktree for each branch before starting a thread
• Graceful handling of non-git repositories with "Initialize git repo" button
• Thread model extended with branch/worktreePath fields for persistence
• Agent sessions use worktree path as cwd when available
• Updated persistence schema to store branch and worktree info

Test Plan

• Right-click "+ New thread" on a git-tracked project to see branch list
• Select a branch to create thread with that branch in metadata
• Click "worktree" button to create isolated worktree before starting thread
• On non-git project, click to see "Not a git repository" with init option
• Verify agent runs in worktree cwd when applicable
• Verify branch/worktree badge displays in chat header

🤖 Generated with Claude Code

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Summary by CodeRabbit

• New Features

  • Branch toolbar beneath chat input to choose branch and environment (Local vs Worktree); environment locks after the first message.
  • Branch picker auto-populates from the repository; toolbar hidden for non-git projects.
  • First message in Worktree mode can create a new worktree and associate it with the thread; branch/worktree badges appear in header and sidebar.

• Tests

  • Extensive unit and integration tests for branch, worktree, checkout, and end-to-end git flows.