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>

CLAUDE.md

@@ -64,7 +64,9 @@ BATTERIES (opt-in siblings, each nil-safe + a default):
   persona/   Agent noun + Storage seam + builtin loader [P4 ~]
              + ToRunnable() bridge to run.RunnableAgent +
              Memory default (host: chatbot/commands/personalization)
-  skill/     rich Skill + SkillStore seam + toml loader  [P4]
+  skill/     Skill noun + LEAN SkillStore (lifecycle/    [P4 ~]
+             versions/schedule — NOT mort's 60-method
+             monster) + ToRunnable + Memory default
   audit/     run.Audit Sink + Writer + queryable Memory  [P4 ✓]
              default (skillaudit Storage iface; GORM stays in mort)
   critic/    two-tier timeout state machine + Escalator  [P4]