Migrate 3D modeling tools to standalone MCP server navi-3d ...

Phase 0 — Fix MCP error propagation:
- McpClient.call_tool now returns (output, is_error) tuple
- McpTool.execute sets success=False when MCP reports isError=True
- Fixes silent failures where MCP errors looked like success

Phase 1 — Create mcp-server-navi-3d:
- New standalone MCP server in mcp-servers/navi-3d/
- Tools: compile_scad, render_stl, lint_scad
- Session-scoped path resolution via SESSION_FILES_DIR env
- Anti-escape validation for security
- Includes tests/unit/test_scad_analyze.py

Phase 2 — Remove from Navi core:
- Deleted navi/tools/{model_3d,render_3d,scad_lint}.py
- Removed from registry.py builtins and navi/tools/__init__.py
- Updated pyproject.toml testpaths to exclude mcp-servers/

Phase 3 — Wire into Navi config:
- Added navi-3d block to mcp_servers.json (SSE on :8002)
- Updated modeler_3d profile: mcp_servers + updated system_prompt
- All old tool names replaced with mcp_navi-3d_* equivalents

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>