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/store.go

@@ -0,0 +1,44 @@
+package skill
+
+import (
+	"context"
+	"errors"
+	"time"
+)
+
+// ErrNotFound is returned when a skill (or version) lookup misses.
+var ErrNotFound = errors.New("skill not found")
+
+// SkillStore is the persistence seam for saved skills. This is the DELIBERATELY
+// LEAN redesign of mort's 60-method skills.Storage: it carries only skill
+// lifecycle (CRUD + visibility), versioning, and scheduling. The KV/file/quota
+// sub-stores that were fused into mort's interface are NOT here — they are the
+// tools/ store seams (KVStorage / FileStorage / QuotaProvider); email recipients
+// and channel grants stay host concerns. A host backs this with its DB; Memory()
+// is the zero-dependency default; contrib/store adds durable SQLite.
+type SkillStore interface {
+	// Initialize prepares storage (idempotent).
+	Initialize(ctx context.Context) error
+
+	// --- lifecycle ---
+	Save(ctx context.Context, s *Skill) error
+	Get(ctx context.Context, id string) (*Skill, error)
+	GetByName(ctx context.Context, ownerID, name string) (*Skill, error)
+	Delete(ctx context.Context, id string) error
+
+	// --- listing / visibility ---
+	ListByOwner(ctx context.Context, ownerID string) ([]Skill, error)
+	ListPublic(ctx context.Context) ([]Skill, error)
+	ListSharedWith(ctx context.Context, memberID string) ([]Skill, error)
+	ListBuiltinByName(ctx context.Context, name string) (*Skill, error)
+	ListChatbotExposed(ctx context.Context) ([]Skill, error)
+
+	// --- scheduling ---
+	ListDueScheduled(ctx context.Context, now time.Time) ([]Skill, error)
+	MarkScheduledRun(ctx context.Context, skillID string, ranAt, nextAt time.Time) error
+
+	// --- versioning ---
+	AppendVersion(ctx context.Context, sv SkillVersion) error
+	ListVersionsBySkill(ctx context.Context, skillID string, limit int) ([]SkillVersion, error)
+	GetVersionByID(ctx context.Context, versionID string) (*SkillVersion, error)
+}