feat(contract-validator): create plugin structure (#185)

Create the basic plugin structure for contract-validator:

Plugin structure:
- plugins/contract-validator/.claude-plugin/plugin.json
- plugins/contract-validator/.mcp.json
- plugins/contract-validator/mcp-servers/contract-validator -> symlink

MCP server scaffolding:
- mcp-servers/contract-validator/mcp_server/server.py (7 placeholder tools)
- mcp-servers/contract-validator/pyproject.toml
- mcp-servers/contract-validator/requirements.txt
- Virtual environment with mcp>=0.9.0

Tools defined (placeholders):
- parse_plugin_interface
- parse_claude_md_agents
- validate_compatibility
- validate_agent_refs
- validate_data_flow
- generate_compatibility_report
- list_issues

Sprint: Sprint 2 - contract-validator Plugin

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

plugins/contract-validator/mcp-servers/contract-validator

@@ -0,0 +1 @@
+../../../mcp-servers/contract-validator