[feature] Animated watch mode

[rwrife]

commit-sprout watch shows the plant gently swaying/growing in a live TUI loop.

Acceptance criteria

• commit-sprout watch opens a minimal full-screen TUI
• Idle sway animation; re-reads git activity on an interval
• Clean exit on q/Ctrl-C; restores terminal
• Low CPU when idle (frame throttling)

[rwrife]

Opened #20 implementing animated watch mode (commit-sprout watch): full-screen alternate-screen TUI, idle sway, interval git re-read, clean q/Ctrl-C exit with terminal restore, and frame throttling for low idle CPU.