chore(F.15): FastMCP 3.x upgrade + legacy cleanup — v3.5.0

- Upgrade fastmcp from >=2.14.0,<3.0.0 to >=3.0.0,<4.0.0
- Remove server_multi.py (legacy multi-endpoint server)
- Remove core/project_manager.py (legacy project manager)
- Refactor HealthMonitor to use SiteManager exclusively
- Replace _tool_manager._tools with tracked _tool_counts dict
- Clean core/__init__.py exports
- Update CHANGELOG, CLAUDE.md, tests
- Auto-format fixes (black + ruff)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_admin_system.py

@@ -1,6 +1,5 @@
 """Tests for Admin System Unification (F.4)."""
 
-
 from core.dashboard.auth import (
     DashboardSession,
     get_session_display_info,