Watering can grace period (#7)

[rwrife]

Closes #7

Adds commit-sprout water — a deliberate escape hatch that buys the plant a grace day for a weekend / PTO — without letting it fake a streak.

Acceptance criteria

• commit-sprout water records a grace day in state (persisted as auditable watered_dates in state.json)
• Grace days are limited/rate-limited: one per calendar day (idempotent), capped at 2 in effect at once, and pruned by any fresh commit so they can't be banked across commits into a permanent green streak
• Status output shows remaining grace and the projected next-wilt date
• Watering is reflected in the health calc only — it shifts the thirsty/wilting thresholds, and never touches the growth stage or the streak number

What changed

• internal/plant: State gains GraceDays; healthFor rewinds the effective dry spell by (clamped) grace; grace is surfaced on PlantState and clamped to [0, MaxGraceDays=2]. Stage/streak are untouched.
• internal/store: persists watered_dates (sorted, de-duped, bounded); EffectiveGrace scopes grace to the current gap (waterings must post-date the last commit and not be in the future); Water enforces per-day idempotency + the cap; FromPlant prunes waterings on/before a fresh commit.
• cmd/water.go: new subcommand with clear applied / already-watered / at-cap messaging; honors --no-save.
• cmd/status.go: adds grace: and wilts: lines, grace-aware.
• README: documents the watering can and the new status lines.

Example

$ commit-sprout status        # 2 days idle
health:  thirsty — 2 days until it wilts
wilts:   2026-07-07
$ commit-sprout water
💧 Watered. The plant can coast one more day.
$ commit-sprout status
grace:   1 day (watered)
health:  healthy — commit today to stay ahead of thirst
wilts:   2026-07-08

Tests

New unit coverage for threshold shifting, clamping, streak-independence, per-day idempotency, cap-refusal, commit-prune, normalize (sort/dedupe/drop-garbage), and save/load round-trip. gofmt/go vet/go build/go test ./... all clean. Also verified end-to-end against a throwaway repo.