feat: pivot to island survival simulation with SQLite persistence

Phase 3 - "The Island" transformation:
- Add SQLAlchemy + SQLite for data persistence (database.py)
- Rewrite models.py with User, Agent, WorldState ORM models
- Refactor engine.py for survival mechanics (energy decay, starvation)
- Implement feed command (10 gold -> +20 energy)
- Auto-seed 3 NPCs on startup (Jack/Luna/Bob)
- Update frontend with agent card view and Chinese UI
- Remove old Boss/Player RPG mechanics
- Add .gitignore for database and cache files
- Fix SQLAlchemy session detachment issue

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,32 @@
+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+.Python
+*.egg
+*.egg-info/
+.eggs/
+
+# Virtual environment
+venv/
+.venv/
+ENV/
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# Database
+*.db
+*.sqlite
+*.sqlite3
+
+# Logs
+*.log
+
+# OS
+.DS_Store
+Thumbs.db