[feature] Wilt notifications — opt-in nudge when plant is about to wilt

[rwrife]

Backlog PLAN.md §8.12 (kept gentle/opt-in per §9). A commit-sprout notify (or --notify) that emits a single nudge when the plant is within a day of wilting, for cron/atuin/precmd hooks.

Pitch: The behavioral nudge only lands if you see it. An opt-in, quiet notify hook lets users wire a daily cron/shell hook that pings them only when the plant is at risk — no daemon, no shaming, no default noise (§9).

Acceptance criteria:

• commit-sprout notify subcommand: exits 0 + prints a one-line warning ONLY when days-until-wilt <= threshold; otherwise silent, exit 0. Non-zero reserved for real errors.
• --threshold (default 1 day) and --quiet/JSON output option for scripting.
• Optional native desktop notification via notify-send/osascript behind a --desktop flag, gracefully skipped if unavailable (never hard-fails).
• No daemon, no background watching (honors §9 'runs on demand only').
• Pure days-until-wilt logic unit-tested in internal/plant.
• README: notify section with a cron + shell-hook snippet, clearly labeled opt-in.
• go build ./... and go test ./... pass.