feat: add worktree sidebar panel

[markwylde]

Summary

• add a Worktrees sidebar pane beneath Git with collapsible linked worktree rows
• show working tree changes under each worktree and color worktree rows by active/dirty state
• add worktree switch, rename, delete, terminal, and reveal actions backed by Git worktree commands

Verification

• npm run build:app
• npm run lint
• git diff --check