Add commit-sprout brag — standup-ready growth one-liner

[rwrife]

Closes #21

Adds commit-sprout brag: a single, standup-ready line summarizing recent growth, driven entirely by real commit data. No new persisted state — reuses the shared runPipeline() (gitstat → plant) and persists afterward like other read commands (honors --no-save).

What's included

• cmd/brag.go — new subcommand + bragLine, a pure formatter over (Activity, PlantState, rememberedPeak, bestStreak). Four shapes:
  • grew: This week your plant grew sprout→leafy on a 5-day streak (14 commits in 7 days). 🌱
  • held steady: ...held steady at leafy...
  • thirsty/wilting: honest revive nudge (with best-streak tail when thirsty)
  • no commits: friendly plant a seed message
• internal/plant.ParseStage — exported inverse of Stage.String (store's unexported parseStage now delegates to it), so the command can resolve the remembered peak stage without an import cycle.
• Registered bragCmd on root.
• Tests — table-driven bragLine + helper coverage in cmd/brag_test.go.
• README — brag example plus post-commit-hook / standup usage snippets.

Acceptance criteria

• one-line window recap: stage transition / held, streak, commit count
• growth phrasing reflects remembered peak vs live stage
• wilting/thirsty brag honestly
• no commits → friendly seed nudge
• plain, script-friendly output (matches status conventions)
• reuses runPipeline(), persists like read commands, respects --no-save
• pure, table-driven unit tests
• README updated
• go build ./... and go test ./... pass

go build, go vet, go test ./..., and gofmt all clean.