[M4] ASCII art + render: stage/health frames, caption, color path

[rwrife]

Closes #4

Implements M4 — ASCII art + render. internal/render now turns a plant.PlantState into a full framed plant, and the root command renders the live plant for your repo instead of the M1 static seedling.

What changed

• render.Frame(PlantState, Options) — the real render. Picks ASCII art by (stage × health) and appends a caption block: stage, streak, last commit (phrased as today / yesterday / N days ago), a days-until-wilt nudge when thirsty/wilting, and the plant's mood line.
• Art stored as data — a map[Stage]map[Health]string table, upright for healthy and progressively drooping for thirsty → wilting, so condition is visible, not just captioned. Easy to add/edit frames.
• Color via lipgloss — foliage color by health (green / yellow / red, magenta bloom accent) with a faint caption. Options.Color gates it.
• Pipe-safe plain path — the plain-ASCII path is used automatically for --no-color, the NO_COLOR env var, and any non-TTY stdout (piped/redirected). cmd/root.go decides via go-isatty; lipgloss's own profile detection is a second safety net. Plain output is guaranteed escape-free (tested).
• render.PromptGlyph — compact one-line glyph stage primitive for M6 prompt mode.
• Root command wired — commit-sprout now runs gitstat.Read → plant.Compute → render.Frame. Handles not-a-repo and no-commits gracefully. Added --no-color; kept the hidden --activity debug bridge.
• Seedling() retained for backward compat (it's the healthy Seed frame; M1 stability test still guards it).

State note

M5 (persistence) isn't wired yet, so a zero plant.State is passed for now — the memory floor behaves correctly from a fresh state, and M5 will load the remembered highest stage/streak at this seam.

Tests

Snapshot-style, plain-text (deterministic via pinned Now/LastCommit):

• every (stage × health) frame exists and is non-blank
• art is distinct across all stages and across all health states per stage (the M4 "done when")
• plain frame contains no ANSI; color path renders without panic
• exact snapshots for healthy-leafy and wilting-tall (art + caption + wilt hint)
• no-commit caption, thirsty countdown wording, streak pluralization, single-line prompt glyph

Verification (Go 1.23.4, matching CI)

• gofmt -l . clean · go vet ./... clean · go build ./... ok · go test ./... -count=1 ok
• Manual smoke: piped / --no-color / NO_COLOR=1 all plain; forced-TTY emits ANSI; --activity still works.

Also refreshed README step 4 with an "Implemented — M4" note mirroring the M2/M3 annotations.