Pests & weeds: reverts spawn cosmetic aphids (#23)

[rwrife]

Closes #23

Introduces gentle negative signals: revert commits spawn cosmetic pests (aphids/weeds) on the plant that you clear by returning to a healthy commit cadence. Purely visual + moral — never lowers stage, streak, or health.

What changed

• internal/gitstat: Activity gains a Reverts int window aggregate. Reverts are detected purely from the conventional Revert "…" commit subject (added %s to the log pretty-format) — no commit content/diffs read (honors PLAN.md §9). Fixture-covered, including prose "revert" false-positive rejection and out-of-window exclusion.
• internal/plant: pestsFor maps Reverts → a bounded Pests int (clamped to MaxPests = 3), pure and unit-tested. Never touches Stage/Streak/Health. Clears after a clean healthy commit today (real work, not just more reverts).
• internal/render: draws pests as % glyphs overlaid on the foliage, with a caption hint. Overlaid on raw art before the color pass so ANSI stays intact; degrades cleanly under --no-color/NO_COLOR/piped output.
• cmd: --no-pests flag disables the overlay entirely (visual-only opt-out).
• README: new "Pests & weeds" section + roadmap entry.

Acceptance criteria

• gitstat.Activity.Reverts parsed from git log, fixture tests
• internal/plant bounded Pests, pure + unit-tested, never lowers Stage/Streak
• internal/render draws pest glyphs, degrades under --no-color
• Pests clear after a clean healthy commit (transition tested)
• --no-pests flag
• README documents spawn + clear
• go build ./... and go test ./... pass

Manually verified end-to-end against a repo with revert commits (pests appear, cap at 3, clear on clean commit, --no-pests suppresses).