feat: Implement Option A - MCP Model Discovery tool

This commit implements the first phase of the MCP server as outlined
in PROJECT_SUMMARY.md Option A: Model Discovery.

New Features:
- Complete .sim file parser (XML-based)
- Expression extraction from .sim and .prt files
- Solution, FEM, materials, loads, constraints extraction
- Structured JSON output for LLM consumption
- Markdown formatting for human-readable output

Implementation Details:
- mcp_server/tools/model_discovery.py: Core parser and discovery logic
  - SimFileParser class: Handles XML parsing of .sim files
  - discover_fea_model(): Main MCP tool function
  - format_discovery_result_for_llm(): Markdown formatter
- mcp_server/tools/__init__.py: Updated to export new functions
- mcp_server/tools/README.md: Complete documentation for MCP tools

Testing & Examples:
- examples/test_bracket.sim: Sample .sim file for testing
- tests/mcp_server/tools/test_model_discovery.py: Comprehensive unit tests
- Manual testing verified: Successfully extracts 4 expressions, solution
  info, mesh data, materials, loads, and constraints

Validation:
- Command-line tool works: python mcp_server/tools/model_discovery.py examples/test_bracket.sim
- Output includes both Markdown and JSON formats
- Error handling for missing files and invalid formats

Next Steps (Phase 2):
- Port optimization engine from P04 Atomizer
- Implement build_optimization_config tool
- Create pluggable result extractor system

References:
- PROJECT_SUMMARY.md: Option A (lines 339-350)
- mcp_server/prompts/system_prompt.md: Model Discovery workflow

mcp_server/tools/__init__.py

@@ -12,9 +12,11 @@ Available tools:
 """
 
 from typing import Dict, Any
+from .model_discovery import discover_fea_model, format_discovery_result_for_llm
 
 __all__ = [
     "discover_fea_model",
+    "format_discovery_result_for_llm",
     "build_optimization_config",
     "start_optimization",
     "query_optimization_status",