P4: skill noun — domain + LEAN SkillStore + ToRunnable + Memory

The skill half of the persona/skill pair, as a clean redesign (not a faithful
lift of mort's 60-method skills.Storage kitchen sink):
- skill.go/skill_version.go/validate.go/inputs.go/schedule.go moved clean; the
  only host couplings severed: llms.IsTierName -> model.IsTierName, and the
  chatbot DefaultChatbotInputName const localized.
- store.go: a DELIBERATELY LEAN SkillStore — skill lifecycle (CRUD + visibility)
  + versioning + scheduling ONLY. The KV/file/quota sub-stores that were fused
  into mort's interface are the tools/ store seams; email/channel grants stay
  host concerns.
- runnable.go: Skill.ToRunnable() lowers a skill into run.RunnableAgent (flat
  tool list, no palette — composition is a host concern); DueAt() helper.
- memory.go: NewMemory() — zero-dep in-process SkillStore (visibility filters,
  newest-first versions).

Tests: ToRunnable mapping, visibility (public/shared/private) listing, version
ordering + lookup. No mort dependency (go.mod tidy clean); core imports ZERO
from skill.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

skill/skill_version.go

@@ -0,0 +1,28 @@
+package skill
+
+import "time"
+
+// SkillVersion is one immutable snapshot of a Skill at the moment it
+// was saved. The skill_versions table is append-only; pruning is by
+// retention policy in PruneOldVersions.
+//
+// Why: edit history with rollback (v3) and the admin pin gate (v3 Phase 4)
+// both need a stable snapshot of the skill at a known version. The Snapshot
+// field carries the FULL Skill struct so a later restore or pin produces
+// the exact agent definition that was saved — system_prompt, tools,
+// schedule, every field — not a synthesized partial snapshot.
+//
+// What: identity (UUID per snapshot) + skill ref + version-string copy +
+// the full Skill payload + audit fields (saved_by, saved_at, edit_summary).
+//
+// Test: see skill_version_test.go for round-trip, list ordering, prune
+// retention, and version-by-number disambiguation coverage.
+type SkillVersion struct {
+	ID          string    // UUID per snapshot (NOT the skill's ID)
+	SkillID     string    // FK to skills.id (conceptually; not enforced by GORM)
+	Version     string    // Skill.Version at save time (semver)
+	Snapshot    Skill     // full Skill struct embedded; serialised as JSON
+	SavedBy     string    // caller member ID (or "" for builtin loader / pre-v3)
+	SavedAt     time.Time // wall-clock save time
+	EditSummary string    // optional human-readable note ("changed model tier", "...")
+}