feat: Implement Phase 1 - Plugin & Hook System

Core plugin architecture for LLM-driven optimization:

New Features:
- Hook system with 6 lifecycle points (pre_mesh, post_mesh, pre_solve, post_solve, post_extraction, custom_objectives)
- HookManager for centralized registration and execution
- Code validation with AST-based safety checks
- Feature registry (JSON) for LLM capability discovery
- Example plugin: log_trial_start
- 23 comprehensive tests (all passing)

Integration:
- OptimizationRunner now loads plugins automatically
- Hooks execute at 5 points in optimization loop
- Custom objectives can override total_objective via hooks

Safety:
- Module whitelist (numpy, scipy, pandas, optuna, pyNastran)
- Dangerous operation blocking (eval, exec, os.system, subprocess)
- Optional file operation permission flag

Files Added:
- optimization_engine/plugins/__init__.py
- optimization_engine/plugins/hooks.py
- optimization_engine/plugins/hook_manager.py
- optimization_engine/plugins/validators.py
- optimization_engine/feature_registry.json
- optimization_engine/plugins/pre_solve/log_trial_start.py
- tests/test_plugin_system.py (23 tests)

Files Modified:
- optimization_engine/runner.py (added hook integration)

Ready for Phase 2: LLM interface layer

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

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

optimization_engine/plugins/__init__.py

@@ -0,0 +1,32 @@
+"""
+Atomizer Plugin System
+
+Enables extensibility through hooks at various stages of the optimization lifecycle.
+
+Hook Points:
+- pre_mesh: Before meshing operations
+- post_mesh: After meshing, before solve
+- pre_solve: Before solver execution
+- post_solve: After solve, before result extraction
+- post_extraction: After result extraction, before objective calculation
+- custom_objectives: Custom objective/constraint functions
+
+Usage:
+    from optimization_engine.plugins import HookManager
+
+    hook_manager = HookManager()
+    hook_manager.register_hook('pre_solve', my_custom_function)
+    hook_manager.execute_hooks('pre_solve', context={'trial_number': 5})
+"""
+
+from .hooks import Hook, HookPoint
+from .hook_manager import HookManager
+from .validators import validate_plugin_code, PluginValidationError
+
+__all__ = [
+    'Hook',
+    'HookPoint',
+    'HookManager',
+    'validate_plugin_code',
+    'PluginValidationError'
+]