feat: agent persistent memory store

[DorianZheng]

Summary

• Adds agent_memory SQLite table (group_jid, key, value, updated_at)
• Three new actions: memory_get, memory_set, memory_list — scoped per group JID
• memory_set with value=null deletes the key
• Full test coverage in src/agent/memory-actions.test.ts

Test plan

• pnpm tsc --noEmit passes
• pnpm test passes with all new tests green
• Manual smoke test: memory_set + memory_get round-trip

Closes item-Wb2vDWih0VXBterEowXl9