add shared knowledge store for cross-agent context sharing

Persistent key-value store in the broker where agents read/write
architectural decisions, discovered patterns, and project context.
Eliminates context drift by giving all agents a shared ground truth
that survives reconnections and crashes.

- New knowledge table in broker (session-scoped, UPSERT-safe)
- 4 broker endpoints: /knowledge/put, get, list, delete
- 3 MCP tools: store_knowledge, query_knowledge, remove_knowledge
- System prompt section 13 instructs agents when to use knowledge
- Launcher tool list updated for agent discovery
- Session delete cleans up knowledge entries
- 14 new tests (knowledge.test.ts) + e2e milestone