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/pre_solve/__init__.py

@@ -0,0 +1 @@
+# pre_solve hooks

optimization_engine/plugins/pre_solve/log_trial_start.py

@@ -0,0 +1,56 @@
+"""
+Example Plugin: Log Trial Start
+
+Simple pre-solve hook that logs trial information.
+
+This demonstrates the plugin API and serves as a template for custom hooks.
+"""
+
+from typing import Dict, Any, Optional
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+def trial_start_logger(context: Dict[str, Any]) -> Optional[Dict[str, Any]]:
+    """
+    Log trial information before solver execution.
+
+    Args:
+        context: Hook context containing:
+            - trial_number: Current trial number
+            - design_variables: Dict of variable values
+            - sim_file: Path to simulation file
+
+    Returns:
+        Dict with logging status
+    """
+    trial_num = context.get('trial_number', '?')
+    design_vars = context.get('design_variables', {})
+
+    logger.info("=" * 60)
+    logger.info(f"TRIAL {trial_num} STARTING")
+    logger.info("=" * 60)
+
+    for var_name, var_value in design_vars.items():
+        logger.info(f"  {var_name}: {var_value:.4f}")
+
+    return {'logged': True, 'trial': trial_num}
+
+
+def register_hooks(hook_manager):
+    """
+    Register this plugin's hooks with the manager.
+
+    This function is called automatically when the plugin is loaded.
+
+    Args:
+        hook_manager: The HookManager instance
+    """
+    hook_manager.register_hook(
+        hook_point='pre_solve',
+        function=trial_start_logger,
+        description='Log trial number and design variables before solve',
+        name='log_trial_start',
+        priority=10  # Run early
+    )