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/list-interfaces.md

@@ -0,0 +1,58 @@
+# /list-interfaces - Show Plugin Interfaces
+
+Display what each plugin in the marketplace produces and accepts.
+
+## Usage
+
+```
+/list-interfaces [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
+   - Read each plugin's README.md
+
+2. **Parse interfaces**:
+   - Extract commands (slash commands offered by plugin)
+   - Extract agents (autonomous agents defined)
+   - Extract tools (MCP tools provided)
+   - Identify tool categories and features
+
+3. **Display summary**:
+   - Table of plugins with command/agent/tool counts
+   - Detailed breakdown per plugin
+   - Tool categories and their contents
+
+## Output Format
+
+```
+| Plugin      | Commands | Agents | Tools |
+|-------------|----------|--------|-------|
+| projman     | 12       | 4      | 26    |
+| data-platform| 7       | 2      | 32    |
+| ...         | ...      | ...    | ...   |
+
+## projman
+- Commands: /sprint-plan, /sprint-start, ...
+- Agents: Planner, Orchestrator, Executor, Code Reviewer
+- Tools: list_issues, create_issue, ...
+```
+
+## Examples
+
+```
+/list-interfaces
+/list-interfaces ~/claude-plugins-work
+```
+
+## Available Tools
+
+Use these MCP tools:
+- `parse_plugin_interface` - Parse individual plugin README
+- `generate_compatibility_report` - Get full interface data (JSON format)