[M5] Persistent state + streaks

[rwrife]

Closes #5

Gives the plant memory between runs by fleshing out internal/store and wiring it into the default render flow.

What changed

• internal/store (was a stub) — full load/save of a versioned JSON state file.
  • XDG-aware path: $XDG_CONFIG_HOME/commit-sprout/state.json, falling back to ~/.config/commit-sprout/state.json (a non-absolute XDG_CONFIG_HOME is ignored).
  • Persists: highest stage ever reached (stored by stable name, e.g. "leafy"), best streak, and last-seen commit hash/time.
  • Atomic writes: temp file in the same dir → Sync → Rename, so a crash or full disk never leaves a corrupt/half-written file. No stray temp files on success.
  • Sane defaults + versioned schema: version field for future migrations; missing files and corrupt/garbled JSON recover to defaults (no error), and the next save heals the file. Legacy version: 0, unknown stage names, and negative streaks are normalized on load.
  • plant interop: State.Plant() projects onto the pure plant.State; State.FromPlant(...) folds a computed result back, ratcheting highest-stage/best-streak upward only (never regresses) and recording the latest commit.
• cmd/root.go — real load → compute → render → save pipeline (replaces the old zero plant.State{}). Persistence is best-effort: load errors fall back to a fresh plant, save errors warn on stderr but never fail the command. New --no-save flag for read-only runs.
• README — marks persistence as implemented (M5) and documents the state file + --no-save.

Checklist (from #5)

• internal/store loads/saves JSON at ~/.config/commit-sprout/state.json (XDG-aware)
• Persist highest stage, current streak, last-seen commit hash/time
• Atomic writes (temp file + rename)
• Sane defaults + migration-friendly versioned schema field
• Wire store into the main flow (load → compute → render → save)
• Tests for round-trip load/save and missing/corrupt file recovery

Testing

• gofmt -l . clean, go vet ./... clean, go build ./... and go test ./... all green.
• 15 new store tests (round-trip, missing/corrupt recovery, normalization, atomicity/no-temp-leak, nested dir creation, XDG + home-fallback paths, Plant/FromPlant ratchet & no-regress).
• End-to-end smoke test: fresh repo with a 4-day streak writes highest_stage: leafy / best_streak: 3; a later stale run with a remembered blooming state correctly renders tall (wilting) (memory floor holds — one stage below peak, not a collapse to seed) and leaves highest-stage/best-streak intact; --no-save leaves the file byte-for-byte untouched.

Not merging — Phase 1 next cycle handles the merge after CI.

Notes / scope

• The schema carries a last_commit_hash field for forward-compat, but gitstat.Activity doesn't currently expose a hash publicly, so it's persisted best-effort (empty for now) and last_commit_time is the field actually populated today. Plumbing a public hash through gitstat would be its own change beyond M5's store scope.