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/skills/validation-rules.md

@@ -0,0 +1,57 @@
+# Skill: Validation Rules
+
+Rules for validating plugin compatibility and agent definitions.
+
+## Compatibility Checks
+
+### 1. Command Name Conflicts
+- No two plugins should have identical command names
+- Severity: ERROR
+
+### 2. Tool Name Overlaps
+- Tools with same name must have same signature
+- Severity: WARNING (may be intentional aliasing)
+
+### 3. Interface Mismatches
+- Producer output types must match consumer input types
+- Severity: ERROR
+
+## Agent Validation
+
+### Tool Reference Check
+1. Parse agent's tool references from workflow steps
+2. Verify each tool exists in available plugins
+3. Report missing or misspelled tool names
+4. Suggest corrections for common mistakes
+
+### Data Flow Validation
+1. Analyze sequence of tools in agent workflow
+2. Verify data producers precede data consumers
+3. Check for orphaned data references
+4. Ensure required data is available at each step
+
+## Severity Levels
+
+| Level | Meaning | Action |
+|-------|---------|--------|
+| ERROR | Will cause runtime failures | Must fix |
+| WARNING | May cause issues | Review needed |
+| INFO | Informational only | No action required |
+
+## Common Issues
+
+| Issue | Cause | Fix |
+|-------|-------|-----|
+| Missing tool | Typo or plugin not installed | Check spelling, install plugin |
+| Data flow gap | Producer not called before consumer | Reorder workflow steps |
+| Name conflict | Two plugins use same command | Rename one command |
+| Orphan reference | Data produced but never consumed | Remove or use the data |
+
+## MCP Tools
+
+| Tool | Purpose |
+|------|---------|
+| `validate_compatibility` | Check two plugins for conflicts |
+| `validate_agent_refs` | Check agent tool references |
+| `validate_data_flow` | Verify data flow sequences |
+| `list_issues` | Filter issues by severity or type |