refactor: extract skills from commands across 8 plugins

Refactored commands to extract reusable skills following the
Commands → Skills separation pattern. Each command is now <50 lines
and references skill files for detailed knowledge.

Plugins refactored:
- claude-config-maintainer: 5 commands → 7 skills
- code-sentinel: 3 commands → 2 skills
- contract-validator: 5 commands → 6 skills
- data-platform: 10 commands → 6 skills
- doc-guardian: 5 commands → 6 skills (replaced nested dir)
- git-flow: 8 commands → 7 skills

Skills contain: workflows, validation rules, conventions,
reference data, tool documentation

Commands now contain: YAML frontmatter, agent assignment,
skills list, brief workflow steps, parameters

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

plugins/contract-validator/commands/check-agent.md

@@ -1,18 +1,10 @@
 # /check-agent - Validate Agent Definition
 
-## Visual Output
-
-When executing this command, display the plugin header:
-
-```
-┌──────────────────────────────────────────────────────────────────┐
-│  ✅ CONTRACT-VALIDATOR · Agent Check                              │
-└──────────────────────────────────────────────────────────────────┘
-```
-
-Then proceed with the validation.
-
-Validate a single agent's tool references and data flow.
+## Skills to Load
+- skills/visual-output.md
+- skills/interface-parsing.md
+- skills/validation-rules.md
+- skills/mcp-tools-reference.md
 
 ## Usage
 
@@ -22,30 +14,26 @@ Validate a single agent's tool references and data flow.
 
 ## Parameters
 
-- `agent_name` (required): Name of the agent to validate (e.g., "Planner", "Orchestrator")
-- `claude_md_path` (optional): Path to CLAUDE.md file. Defaults to `./CLAUDE.md`
+- `agent_name` (required): Agent to validate (e.g., "Planner", "Orchestrator")
+- `claude_md_path` (optional): Path to CLAUDE.md. Defaults to `./CLAUDE.md`
 
 ## Workflow
 
-1. **Parse agent definition**:
-   - Locate agent in CLAUDE.md (Four-Agent Model table or Agents section)
-   - Extract responsibilities, tool references, workflow steps
+1. **Display header** per `skills/visual-output.md`
 
-2. **Validate tool references**:
-   - Check each referenced tool exists in available plugins
-   - Report missing or misspelled tool names
-   - Suggest corrections for common mistakes
+2. **Parse agent** per `skills/interface-parsing.md`
+   - Use `parse_claude_md_agents` to extract agent definition
+   - Get responsibilities, tool references, workflow steps
 
-3. **Validate data flow**:
-   - Analyze sequence of tools in agent workflow
-   - Verify data producers precede data consumers
-   - Check for orphaned data references
+3. **Validate** per `skills/validation-rules.md`
+   - Use `validate_agent_refs` - check all tools exist
+   - Use `validate_data_flow` - verify producer/consumer order
 
 4. **Report findings**:
-   - List all tool references found
-   - List any missing tools
-   - Data flow validation results
-   - Suggestions for improvement
+   - Tool references found
+   - Missing tools (with suggestions)
+   - Data flow issues
+   - Recommendations
 
 ## Examples
 
@@ -54,10 +42,3 @@ Validate a single agent's tool references and data flow.
 /check-agent Orchestrator ./CLAUDE.md
 /check-agent data-analysis ~/project/CLAUDE.md
 ```
-
-## Available Tools
-
-Use these MCP tools:
-- `validate_agent_refs` - Check agent tool references exist
-- `validate_data_flow` - Verify data flow through agent sequence
-- `parse_claude_md_agents` - Parse all agents from CLAUDE.md