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/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", "...")
+}