[feature] sp pin/unpin — exempt scratches from reaping

[rwrife]

Pitch

Backlog item #9. Sometimes a scratch matters more than its TTL implies, but it doesn't deserve the working tree yet (sp promote). Today the only way to keep it is an absurd --ttl 9999d. Add a first-class pin so a scratch is exempt from sp reap without lying about its lifespan.

Behavior

• sp pin <id> — mark a scratch as pinned. Pinned scratches are never moved to the morgue by sp reap, regardless of expiry.
• sp unpin <id> — clear the pin; normal TTL rules resume.
• sp ls shows a pin marker (e.g. 📌 in TTY, PIN in plain/--json) and pinned scratches sort/display as such.
• sp reap (and --dry-run) skips pinned scratches and reports how many were skipped for being pinned.
• Pin state persists in the index (Scratch.Pinned bool, omitempty).

Acceptance criteria

• Scratch model gains a Pinned field (JSON omitempty), backward-compatible with existing indexes.
• sp pin <id> / sp unpin <id> commands registered on root, with tombstone-flavored confirmations.
• sp reap and sp reap --dry-run exclude pinned scratches and note the skip count.
• sp ls (table, plain, and --json) surfaces pin state.
• Unit tests: pin round-trips through the index; reap skips pinned; ls renders the marker.
• README updated with pin/unpin usage.

[rwrife]

Opened #37 implementing pin/unpin (model field, store+reap exemption, pin/unpin commands, ls table/plain/--json markers, reap skip reporting, tests, README).