P4b: skill noun + contrib/store (SQLite for budget/persona/skill/audit)

Merges the skill half of the persona/skill pair plus the second nested module.
(Squashed onto main from phase-4b-skill; the audit/budget/persona batteries it
was stacked on already landed via the P4 merge.)

- skill/: clean-redesign Skill noun + LEAN SkillStore (lifecycle/versions/
  schedule only) + ToRunnable + Memory default.
- contrib/store/: separate go.mod carrying modernc.org/sqlite, so the driver
  never enters the core go.sum. db.Budget()/Personas()/Skills()/Audit() back
  all four store seams (JSON-blob + indexed columns; round-trip tested).
  Includes the verified gadfly #5 fixes (AppendVersion tx+UNIQUE+error,
  Mark*ScheduledRun atomic json_set, busy_timeout, NaN guard).
- CI: builds + tests the nested module and asserts it owns the sqlite driver.

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)
+}