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>

.gitea/workflows/ci.yml

@@ -103,3 +103,15 @@ jobs:
             exit 1
           fi
           echo "OK: core go.sum is free of host/DB dependencies."
+
+      - name: contrib/store (nested SQLite module — isolated from core)
+        run: |
+          # contrib/store is a SEPARATE module carrying modernc.org/sqlite; the
+          # core's `go test ./...` doesn't reach it. Build + test it on its own,
+          # and confirm it DOES carry the driver the core forbids (proof the
+          # split works: persistence lives here, not in the core go.sum).
+          cd contrib/store
+          go build ./...
+          go test -race -count=1 -timeout 5m ./...
+          grep -qE 'modernc.org/sqlite' go.sum || { echo "ERROR: contrib/store should carry the sqlite driver"; exit 1; }
+          echo "OK: contrib/store builds, tests pass, and owns the SQLite dep."