Refactor: move tool helper modules into _internal subpackage ...

Moves non-tool infrastructure out of navi/tools/ root so that only
actual tool classes live there:

  base.py           → _internal/base.py
  loader.py         → _internal/loader.py
  middleware.py     → _internal/middleware.py
  logging_middleware.py → _internal/logging_middleware.py
  _time_parser.py   → _internal/time_parser.py

All imports updated across core/, api/, mcp/, tools/, and tests/.
No proxy files remain in navi/tools/ root.

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

fix: wire test_mcp_tool into MCP manager startup injection ...

The startup loop in main.py assigns _mcp_manager only to tools that
have the attribute, but test_mcp_tool never declared it and mcp_status
used _manager instead of _mcp_manager — so both received None forever.

Changes:
- test_mcp_tool: add __init__(mcp_manager) with _mcp_manager, fallback
  to module-level import if startup wiring skipped
- mcp_status: rename _manager → _mcp_manager, same fallback
- registry.py: register create_mcp_server and test_mcp_tool builtins
- main.py: include test_mcp_tool in startup wiring loop
- client.py: add 5s timeout to disconnect cleanup

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>