M6: tombstone-flavored personality for command confirmations

[rwrife]

Refs #6 (M6 — Polish + personality + ship v0.1.0)

What & why

M6's first acceptance criterion asks for "tombstone-flavored personality in user-facing messages (tasteful, not noisy)." The render layer (ls, --morgue, reap, doctor) already had that voice, but the direct command confirmation lines were still clinical. This brings them in line.

Changes

• sp new → created scratch <id> (name) — living on borrowed time, expires in ~7d (new pure helpers lifespanNote / humanCountdown).
• sp open → … — back on the slab
• sp rm → buried scratch <id> (name) in the morgue — not gone, just resting; restore with \sp resurrect ``
• sp resurrect → … — it claws its way out of the morgue and is live again
• sp promote → … — it escapes the reaper and joins the working tree
• internal/cli/personality_test.go pins the new flavor and guards the stable substrings other tests/scripts depend on (created scratch, morgue, live again, promoted scratch).
• README sp rm example refreshed to match.

Guarantees

• Every confirmation still leads with its existing anchor; no --json/scripting output touched (personality lives only on human confirmation lines).
• go build, go vet, gofmt -l, and the full go test ./... suite are all green.

Sample

created scratch 86318dcd (demo) — living on borrowed time, expires in ~6d
opened scratch a7e3191a (quick) — back on the slab
buried scratch a7e3191a (quick) in the morgue — not gone, just resting; restore with `sp resurrect a7e3191a`
resurrected scratch a7e3191a (quick) — it claws its way out of the morgue and is live again
promoted scratch a7e3191a (quick) → quick.md — it escapes the reaper and joins the working tree

Deliberately NOT in this PR

The remaining M6 item — tagging and publishing the v0.1.0 release (which triggers the GoReleaser release workflow and publishes artifacts) — is a maintainer action and is intentionally left for a human to cut, not automated here. #6 stays open until that ships.