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/runnable.go

@@ -0,0 +1,35 @@
+package skill
+
+import (
+	"time"
+
+	"gitea.stevedudenhoeffer.com/steve/executus/run"
+)
+
+// ToRunnable lowers a saved Skill into the kernel's run.RunnableAgent DTO, so
+// run.Executor can run a skill WITHOUT importing this battery (the inversion of
+// mort's skillexec running a skills.Skill). Maps the static shape only; the
+// skill's input schema → prompt rendering, palette resolution, audit, etc. are
+// supplied separately (the host renders inputs into the input string and wires
+// run.Ports). A skill exposes a flat tool list (no SkillPalette/SubAgentPalette
+// — composition is a host concern), so those stay empty.
+func (s *Skill) ToRunnable() run.RunnableAgent {
+	return run.RunnableAgent{
+		ID:            s.ID,
+		Name:          s.Name,
+		SystemPrompt:  s.SystemPrompt,
+		ModelTier:     s.ModelTier,
+		MaxIterations: s.MaxIterations,
+		MaxRuntime:    s.MaxRuntime,
+		LowLevelTools: s.Tools,
+	}
+}
+
+// DueAt reports whether a scheduled skill is due at now (cron empty => never).
+// Convenience for a host scheduler that doesn't want to re-parse the cron.
+func (s *Skill) DueAt(now time.Time) bool {
+	if s.Schedule == "" || s.NextRunAt.IsZero() {
+		return false
+	}
+	return !s.NextRunAt.After(now)
+}