[feature] sp stats — fun store metrics #32

Pitch

Backlog item #11. A little personality + genuinely useful hygiene insight. sp stats prints fun/useful metrics about the store: how much rot you've avoided, your oldest survivor, your reaping mortality rate.

Behavior

sp stats reports:

• Live scratch count and total bytes.
• Morgue count and bytes (recoverable), plus grace-window status.
• Oldest living scratch ("oldest survivor") and its age.
• Total scratches ever created, total ever reaped/hard-deleted (if tracked; otherwise derive from what's available and label estimates honestly).
• Bytes "saved from rotting in /tmp" — total bytes that passed through the store.
• Tag breakdown (top N tags by count).
• Pinned count (if #pin lands first; otherwise skip gracefully).

Support --json for scripting, consistent with ls --json / doctor --json.

Acceptance criteria

• sp stats command registered on root.
• Computes metrics purely from the index + store on disk (no new persistent counters unless trivially added atomically).
• Human-readable output with tombstone/personality flavor; --json machine output mirrors it.
• Handles empty store gracefully (no scratches → friendly zero-state, not a crash).
• Unit tests covering: empty store, mixed live/morgue, oldest-survivor selection, tag breakdown.
• README updated with a sp stats example.