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>

contrib/store/skill_store_test.go

@@ -0,0 +1,72 @@
+package store
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"gitea.stevedudenhoeffer.com/steve/executus/skill"
+)
+
+func TestSQLiteSkillStore(t *testing.T) {
+	ctx := context.Background()
+	db, err := Open(":memory:")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	st := db.Skills()
+	if err := st.Initialize(ctx); err != nil {
+		t.Fatal(err)
+	}
+
+	pub := &skill.Skill{ID: "a", Name: "pub", OwnerID: "o1", Visibility: skill.VisibilityPublic,
+		Tools: []string{"summarize"}, ExposeAsChatbotTool: true}
+	shared := &skill.Skill{ID: "b", Name: "shr", OwnerID: "o1", Visibility: skill.VisibilityShared, SharedWith: []string{"bob"}}
+	if err := st.Save(ctx, pub); err != nil {
+		t.Fatal(err)
+	}
+	if err := st.Save(ctx, shared); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := st.Get(ctx, "a")
+	if err != nil || len(got.Tools) != 1 || !got.ExposeAsChatbotTool {
+		t.Fatalf("round-trip: %v %+v", err, got)
+	}
+	if ps, _ := st.ListPublic(ctx); len(ps) != 1 || ps[0].ID != "a" {
+		t.Errorf("ListPublic = %+v", ps)
+	}
+	if ss, _ := st.ListSharedWith(ctx, "bob"); len(ss) != 1 || ss[0].ID != "b" {
+		t.Errorf("ListSharedWith(bob) = %+v", ss)
+	}
+	if ss, _ := st.ListSharedWith(ctx, "carol"); len(ss) != 0 {
+		t.Errorf("ListSharedWith(carol) should be empty: %+v", ss)
+	}
+	if ce, _ := st.ListChatbotExposed(ctx); len(ce) != 1 {
+		t.Errorf("ListChatbotExposed = %d, want 1", len(ce))
+	}
+
+	// Versions newest-first + by id.
+	st.AppendVersion(ctx, skill.SkillVersion{ID: "v1", SkillID: "a", Version: "1.0.0"})
+	st.AppendVersion(ctx, skill.SkillVersion{ID: "v2", SkillID: "a", Version: "1.1.0"})
+	vs, _ := st.ListVersionsBySkill(ctx, "a", 10)
+	if len(vs) != 2 || vs[0].ID != "v2" {
+		t.Errorf("versions newest-first: %+v", vs)
+	}
+	if gv, err := st.GetVersionByID(ctx, "v1"); err != nil || gv.Version != "1.0.0" {
+		t.Errorf("GetVersionByID: %v %+v", err, gv)
+	}
+
+	// Scheduling.
+	now := time.Now().UTC()
+	cron := &skill.Skill{ID: "c", Name: "cron", OwnerID: "o1", Schedule: "0 * * * *", NextRunAt: now.Add(-time.Minute)}
+	st.Save(ctx, cron)
+	if due, _ := st.ListDueScheduled(ctx, now); len(due) != 1 || due[0].ID != "c" {
+		t.Fatalf("ListDueScheduled = %+v", due)
+	}
+	st.MarkScheduledRun(ctx, "c", now, now.Add(time.Hour))
+	if due, _ := st.ListDueScheduled(ctx, now); len(due) != 0 {
+		t.Errorf("after MarkScheduledRun nothing due: %+v", due)
+	}
+}