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>

Add memory_save tool for proactive fact persistence ...

Navi previously had no way to write to memory mid-conversation — she
could only search and forget. Facts were extracted automatically after
sessions went idle for 30+ min, so important context shared by the user
could be lost or delayed.

- New MemorySaveTool (navi/tools/memory_save.py): upsert a fact by
  category/key/value; overwrites existing key so no separate forget needed
- Registered as builtin alongside memory_search/memory_forget
- Added to all three profiles (secretary, server_admin, smart_home)
- persona.txt: explicit "call memory_save immediately when..." guidance
  so Navi saves stable facts as they arrive, not only post-session

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>