[feature] Pests & weeds — reverts/force-pushes spawn aphids to clear #23

Pitch

Backlog item #6. Introduce gentle negative signals: messy git behavior (reverts, force-pushes, unusually huge red-diff days) spawns visible "pests" (aphids/weeds) on the plant that you clear by returning to healthy commit cadence. Turns the plant into a fuller behavioral mirror without becoming a nag.

Why it fits

• Extends internal/gitstat to surface a few extra signals (revert commits, force-push markers) from data git already exposes, then internal/plant maps them to a bounded pest count, and internal/render draws them. Clean separation, each layer testable.
• Keeps the "life you're responsible for" nudge from PLAN.md §3 stronger than a flat chart.

Design notes / scope guard

• Detect reverts via commit-message convention (^Revert ") and/or git log --grep. Force-push detection is best-effort from reflog if available; if unavailable, skip gracefully (no hard dependency).
• Pests are cosmetic + moral, never destroy growth: they cap at a small number and clear after a healthy commit day. No stage/streak penalty in v1 (stay gentle, opt-in for anything harsher).
• Do NOT read commit content/diffs for anything beyond a size heuristic (respect PLAN.md §9 "no reading commit content").

Acceptance criteria

• gitstat.Activity gains a small, well-documented signal (e.g. Reverts int within window) parsed purely from git log output; covered by fixture tests.
• internal/plant maps signals → a bounded Pests int (clamped), pure and unit-tested; never lowers Stage/Streak in v1.
• internal/render draws pests as small ASCII glyphs (e.g. %) on/around the plant, degrading under --no-color.
• Pests clear after a clean healthy commit (state transition tested).
• --no-pests flag to disable entirely for people who don't want it.
• README documents what spawns pests and how to clear them.
• go build ./... and go test ./... pass.