feat(memory): persistent memory layer with remember/recall CodeAct API

Add a memories table (survives index rebuilds) for agent-written
  conclusions with provenance (session, message range, project). The agent
  writes markdown files via Write tool (user-approved), then registers
  them via a --remember CodeAct script with remember(). Recall via
  memories() in --query scripts, filtered by project/session/time.

  Separates query (read-only, assertReadOnlySql) from remember (write)
  execution contexts in runtime.mjs.

.gitignore

@@ -1,3 +1,4 @@
 .DS_Store
 plans/
-.skillopt-backups
+.skillopt-backups
+tests/