feat(contract-validator): create commands (#189)

Add 3 user-facing commands:
- /validate-contracts: Full marketplace compatibility validation
- /check-agent: Validate single agent definition
- /list-interfaces: Show plugin interfaces summary

Each command documents usage, workflow, parameters, and available
MCP tools for implementation.

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

plugins/contract-validator/commands/validate-contracts.md

@@ -0,0 +1,50 @@
+# /validate-contracts - Full Contract Validation
+
+Run comprehensive cross-plugin compatibility validation for the entire marketplace.
+
+## Usage
+
+```
+/validate-contracts [marketplace_path]
+```
+
+## Parameters
+
+- `marketplace_path` (optional): Path to marketplace root. Defaults to current project root.
+
+## Workflow
+
+1. **Discover plugins**:
+   - Scan plugins directory for all plugins with `.claude-plugin/` marker
+   - Parse each plugin's README.md to extract interface
+
+2. **Run compatibility checks**:
+   - Perform pairwise compatibility validation between all plugins
+   - Check for command name conflicts
+   - Check for tool name overlaps
+   - Identify interface mismatches
+
+3. **Validate CLAUDE.md agents**:
+   - Parse agent definitions from CLAUDE.md
+   - Validate all tool references exist
+   - Check data flow through agent sequences
+
+4. **Generate report**:
+   - Summary statistics (plugins, commands, tools, issues)
+   - Detailed findings by severity (error, warning, info)
+   - Actionable suggestions for each issue
+
+## Examples
+
+```
+/validate-contracts
+/validate-contracts ~/claude-plugins-work
+```
+
+## Available Tools
+
+Use these MCP tools:
+- `generate_compatibility_report` - Generate full marketplace report
+- `list_issues` - Filter issues by severity or type
+- `parse_plugin_interface` - Parse individual plugin interface
+- `validate_compatibility` - Check two plugins for conflicts