Animated watch mode (#10)

[rwrife]

Closes #10

Adds commit-sprout watch: a minimal full-screen TUI that renders the plant and gently sways while re-reading git activity on an interval.

What changed

• New cmd/watch.go: alternate-screen loop with raw-mode input (via golang.org/x/sys/unix), signal handling, and ANSI escapes — no new heavy deps.
• Idle sway animation (triangle-wave indent) between reads; git re-read on --interval (default 5s).
• Clean exit on q / Q / Ctrl-C or SIGINT/SIGTERM; alternate screen, cursor, and termios always restored via defers.
• Low idle CPU: frames throttled (~700ms sway step) and redraw only on change.
• Refuses to start when stdout is not a TTY; degrades gracefully outside a repo.
• Reuses the shared runPipeline; read-only (never persists state).
• Tests for the pure helpers (swayIndent, composeScreen); README + roadmap updated.

Acceptance criteria

• 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 + redraw-on-change)

gofmt/vet/build/test all green locally.