[feature] commit-sprout card — export plant as SVG README badge

[rwrife]

Closes #27

What changed

Adds a card subcommand that exports the current plant as a self-contained SVG so users can embed a living-ish plant badge in their README — an on-brand answer to the green-squares contribution graph.

Command

• commit-sprout card [--out plant.svg] — new cobra subcommand.
• --out - streams the SVG to stdout; default writes plant.svg in cwd via an atomic write (temp file + rename), mirroring internal/store.Save.
• Read-only: exporting a card never persists plant state.
• --no-color (now a persistent root flag) / NO_COLOR yields a mono card; otherwise foliage color honors the theme (green / amber / red, magenta bloom).

Rendering (internal/render/card.go)

• New exported render.Card(ps, CardOptions) builds a complete <?xml?>-prefixed <svg> document.
• Art rows mirror the exact terminal plain art (with pests overlaid) as monospace <tspan> rows; the caption reuses the shared caption() block for stage/streak/last-commit/hints parity.
• No external fonts or assets; generic monospace family; all glyphs inlined and XML-escaped.

Tests (internal/render/card_test.go)

• Asserts a valid <svg> root + full XML well-formedness.
• Asserts expected glyph rows for a known state appear as tspans.
• Table test that color path emits the right foliage hex per health/bloom.
• Verifies --no-color produces a mono card (neutral ink, no themed foliage).

Docs

• README: quick-interface line + a new "README badge (SVG card)" section with usage, embed snippet, and regeneration tips (hook / cron / CI).

Acceptance criteria

• card subcommand with --out
• Self-contained SVG mirroring ASCII art + stage/streak caption
• --out - to stdout; default atomic plant.svg
• Theme color path + --no-color mono
• Unit test: valid SVG root + expected glyph rows
• README card section + embed snippet
• go build ./... and go test ./... pass

Notes / scope

• --no-color was promoted to a persistent flag so subcommands (including card) can honor it; behavior for the root command is unchanged.
• Season/Holiday dressing is intentionally not layered onto the card yet (kept legible + diff-stable); reserved for a follow-up.