Inject MCP server instructions into system prompt ...

- McpClient: collect instructions from MCP initialize handshake
- McpServerConfig: add 'instructions' field for Navi-side overlay
- McpManager.get_instructions(): merge server + config instructions
- ContextBuilder: new _mcp_context_msg() injects MCP server
  descriptions into every LLM context as a system message
- Agent passes mcp_manager to ContextBuilder
- mcp_servers.json: add overlay instructions for gnexus-book

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add MCP server support and fix memory tools user isolation ...

MCP integration:
- New navi/mcp/ package: client, manager, config, tools
- ToolRegistry learns register_external() for MCP tools
- reload_tools reconnects MCP servers on hot reload
- New built-in mcp_status tool
- Startup/shutdown wiring for MCP connections
- 12 new tests (unit + integration with real stdio server)

Memory tools fix:
- memory, memory_save, memory_search, memory_forget now read
  current_user_id from tool context and pass it to MemoryStore
- Fixes invisible facts for authenticated users

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>