fix: address verified gadfly P5/#5 findings (contrib/store concurrency)

All 3 cloud models converged on real concurrency bugs in the SQLite stores:

- AppendVersion (HIGH): the seq key was `SELECT MAX(seq)+1` then INSERT in two
  un-transacted statements with a NON-unique index, AND the Scan error was
  swallowed (seq stayed 0 on failure). Concurrent appends could both land the
  same seq, silently breaking newest-first ordering. Now: one transaction, the
  Scan error is propagated, the (skill_id, seq) index is UNIQUE (the loser of a
  race fails loudly), and an empty SkillID is rejected.
- MarkScheduledRun / MarkAgentScheduledRun (all 3): replaced the Get→mutate→Save
  read-modify-write (lost-update window) with a single atomic UPDATE using
  json_set, so a concurrent Mark/edit can't clobber it. json_set keeps the JSON
  blob's NextRunAt/LastScheduledRunAt consistent with the indexed column;
  RFC3339Nano matches Go's time encoding so the blob still round-trips (tested).
- Open: actually applies PRAGMA busy_timeout=5000 (the doc advertised it but it
  was never set) — a contended writer waits instead of erroring SQLITE_BUSY.
- budgetStore.Add: rejects NaN/Inf secondsUsed (would irrecoverably poison the
  column).

Triaged-but-kept: plaintext webhook secret (documented design, high-entropy URL
key, pre-existing); SQL()/free-form `where` helpers (no untrusted input reaches
them — defense-in-depth notes only).

Core go.sum still free of host/DB deps; contrib/store green (incl. a json_set
blob-round-trip test).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

contrib/store/persona_store_test.go

@@ -69,3 +69,38 @@ func TestSQLitePersonaStore(t *testing.T) {
 		t.Errorf("GetAgent after delete = %v, want ErrNotFound", err)
 	}
 }
+
+// TestMarkAgentScheduledRunBlobRoundTrips guards the json_set atomic update:
+// the JSON blob must stay parseable and reflect the new scheduled times.
+func TestMarkAgentScheduledRunBlobRoundTrips(t *testing.T) {
+	ctx := context.Background()
+	db, _ := Open(":memory:")
+	defer db.Close()
+	st := db.Personas()
+	st.InitializeAgentStorage(ctx)
+	start := time.Now().UTC()
+	a := &persona.Agent{ID: "m1", Name: "n", OwnerID: "o", Schedule: "0 * * * *"}
+	a.NextRunAt = &start
+	if err := st.SaveAgent(ctx, a); err != nil {
+		t.Fatal(err)
+	}
+	ran := start
+	next := start.Add(time.Hour)
+	if err := st.MarkAgentScheduledRun(ctx, "m1", ran, next); err != nil {
+		t.Fatal(err)
+	}
+	got, err := st.GetAgent(ctx, "m1") // blob must still unmarshal
+	if err != nil {
+		t.Fatalf("GetAgent after json_set Mark failed (blob corrupt?): %v", err)
+	}
+	if got.NextRunAt == nil || !got.NextRunAt.Equal(next) {
+		t.Errorf("blob NextRunAt = %v, want %v", got.NextRunAt, next)
+	}
+	if got.LastScheduledRunAt == nil || !got.LastScheduledRunAt.Equal(ran) {
+		t.Errorf("blob LastScheduledRunAt = %v, want %v", got.LastScheduledRunAt, ran)
+	}
+	// Unknown id -> ErrNotFound.
+	if err := st.MarkAgentScheduledRun(ctx, "nope", ran, next); err != persona.ErrNotFound {
+		t.Errorf("Mark(unknown) = %v, want ErrNotFound", err)
+	}
+}