[feature] Bloom on milestones — tags/releases spawn flowers

[rwrife]

Backlog PLAN.md §8.5. Big commit days, git tags, or releases should trigger a bloom (flowers on the plant) as a positive milestone signal.

Pitch: Right now growth is smooth stage progression. Tags/releases are the developer's real 'milestone' moments — the plant should visibly celebrate them with a bloom overlay, distinct from steady leafy growth.

Acceptance criteria:

• internal/gitstat: Activity gains a Tags int (or RecentTag bool) window aggregate, parsed from git tag --points-at/git log --decorate or git for-each-ref timestamps — no commit content read (honors §9). Fixture-tested.
• Configurable 'big day' threshold (commits in a single day) also triggers bloom; pure + unit-tested in internal/plant.
• internal/plant: bounded Blooms/bloom flag, never lowers stage/streak/health; clears/fades after N days.
• internal/render: flower glyphs overlaid on foliage, degrade under --no-color/NO_COLOR/piped.
• --no-bloom opt-out flag.
• README section + roadmap tick.
• go build ./... and go test ./... pass.