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_test.go

@@ -0,0 +1,57 @@
+package skill
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+func TestSkillToRunnable(t *testing.T) {
+	s := &Skill{
+		ID: "s1", Name: "summarizer", SystemPrompt: "summarize well", ModelTier: "fast",
+		MaxIterations: 4, MaxRuntime: 20 * time.Second, Tools: []string{"summarize", "now"},
+	}
+	r := s.ToRunnable()
+	if r.ID != "s1" || r.ModelTier != "fast" || r.MaxIterations != 4 || len(r.LowLevelTools) != 2 {
+		t.Fatalf("ToRunnable mapping wrong: %+v", r)
+	}
+	// A skill exposes a flat tool list, not a palette.
+	if len(r.SkillPalette) != 0 || len(r.SubAgentPalette) != 0 {
+		t.Errorf("skill should have empty palettes, got %+v", r)
+	}
+}
+
+func TestMemoryStoreVisibilityAndVersions(t *testing.T) {
+	ctx := context.Background()
+	m := NewMemory()
+	pub := &Skill{ID: "a", Name: "pub", OwnerID: "o1", Visibility: VisibilityPublic}
+	shared := &Skill{ID: "b", Name: "shr", OwnerID: "o1", Visibility: VisibilityShared, SharedWith: []string{"bob"}}
+	priv := &Skill{ID: "c", Name: "prv", OwnerID: "o1", Visibility: VisibilityPrivate}
+	for _, s := range []*Skill{pub, shared, priv} {
+		if err := m.Save(ctx, s); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if ps, _ := m.ListPublic(ctx); len(ps) != 1 || ps[0].ID != "a" {
+		t.Errorf("ListPublic = %+v", ps)
+	}
+	if ss, _ := m.ListSharedWith(ctx, "bob"); len(ss) != 1 || ss[0].ID != "b" {
+		t.Errorf("ListSharedWith(bob) = %+v", ss)
+	}
+	if ss, _ := m.ListSharedWith(ctx, "carol"); len(ss) != 0 {
+		t.Errorf("ListSharedWith(carol) should be empty, got %+v", ss)
+	}
+	if all, _ := m.ListByOwner(ctx, "o1"); len(all) != 3 {
+		t.Errorf("ListByOwner = %d, want 3", len(all))
+	}
+	// Versions: newest-first, fetchable by id.
+	m.AppendVersion(ctx, SkillVersion{ID: "v1", SkillID: "a", Version: "1.0.0"})
+	m.AppendVersion(ctx, SkillVersion{ID: "v2", SkillID: "a", Version: "1.1.0"})
+	vs, _ := m.ListVersionsBySkill(ctx, "a", 10)
+	if len(vs) != 2 || vs[0].ID != "v2" {
+		t.Errorf("versions newest-first wrong: %+v", vs)
+	}
+	if got, err := m.GetVersionByID(ctx, "v1"); err != nil || got.Version != "1.0.0" {
+		t.Errorf("GetVersionByID: %v %+v", err, got)
+	}
+}