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/consts.go

@@ -0,0 +1,6 @@
+package skill
+
+// DefaultChatbotInputName is the input-param name a chatbot-exposed skill
+// receives the user's message under when its schema doesn't name one. Moved
+// from mort's chatbot_provider.go (a host concern) as a host-agnostic default.
+const DefaultChatbotInputName = "request"