feat: git worktree support + branch creation

[JanSmrcka]

Summary

Add worktree picker mode (w key) and branch creation (n in branch picker) to differ's TUI.

Worktree support

• w opens worktree picker (list, switch, create, remove)
• Switching replaces internal Repo pointer → all ops auto-target new worktree
• New worktrees created at .worktrees/{branch} (configurable via worktree_dir config)
• Remove requires double-press d, errors on dirty worktrees (no --force)

Branch creation

• n in branch picker opens text input for new branch name
• Creates branch via git branch <name>, refreshes list

Keybindings

Worktree picker: j/k nav, Enter switch, n new, d remove, esc cancel
Branch picker: n create new branch

Technical approach

• New git methods: ListWorktrees, AddWorktree, AddWorktreeNewBranch, RemoveWorktree, CreateBranch
• New modeWorktreePicker following branch picker pattern
• WorktreeDir config field (default .worktrees/{branch})
• Closes feat: branch creation from within differ #20

Plan

See .claude/plans/moonlit-leaping-tower.md for detailed implementation plan.