feat(desktop): show branch/worktree in window title to distinguish instances

[siracusa5]

Problem

When multiple instances of the desktop app are running (e.g. from different worktrees or branches), there's no way to tell which is which. Both windows show the same title bar.

Expected behavior

The window title should include an identifier — the git branch name or worktree path — so that when multiple instances are open, the user can tell them apart at a glance.

Examples:

• Harness Kit — feat/aperant-board-replication
• Harness Kit — main
• Harness Kit — .worktrees/feat/some-feature

Implementation notes

• Use Tauri's Window::set_title() on startup
• Detect the current branch via git rev-parse --abbrev-ref HEAD or from the worktree path
• Only append the identifier when it's not main (or always show it for clarity)