Add Reveal in Sidebar command

[sbertix]

Requires #183

Summary

• Adds a Reveal in Sidebar action (default Cmd+Shift+E) that shows the sidebar if hidden, expands the repository section containing the selected worktree if collapsed, and focuses the worktree row.
• Registers the shortcut in AppShortcuts (configurable via Settings > Shortcuts).
• Adds 3 unit tests covering: expand collapsed repo, no-op without selection, and keeping other repos collapsed.

Test plan

• With the sidebar hidden, press Cmd+Shift+E — sidebar should appear and the selected worktree should be visible.
• With the sidebar visible but the repo section collapsed, press Cmd+Shift+E — repo section should expand to reveal the worktree.
• With multiple repos, verify only the selected worktree's repo is expanded; other collapsed repos remain collapsed.
• Verify the shortcut appears in Settings > Shortcuts under "Sidebar" and can be customized.
• Run make test — all tests pass.