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/data-platform/commands/explain.md

@@ -1,18 +1,14 @@
 # /explain - dbt Model Explanation
 
+## Skills to Load
+- skills/dbt-workflow.md
+- skills/lineage-analysis.md
+- skills/mcp-tools-reference.md
+- skills/visual-header.md
+
 ## Visual Output
 
-When executing this command, display the plugin header:
-
-```
-┌──────────────────────────────────────────────────────────────────┐
-│  📊 DATA-PLATFORM · Model Explanation                             │
-└──────────────────────────────────────────────────────────────────┘
-```
-
-Then proceed with the explanation.
-
-Explain a dbt model's purpose, dependencies, and SQL logic.
+Display header: `DATA-PLATFORM - Model Explanation`
 
 ## Usage
 
@@ -22,24 +18,10 @@ Explain a dbt model's purpose, dependencies, and SQL logic.
 
 ## Workflow
 
-1. **Get model info**:
-   - Use `dbt_lineage` to get model metadata
-   - Extract description, tags, materialization
-
-2. **Analyze dependencies**:
-   - Show upstream models (what this depends on)
-   - Show downstream models (what depends on this)
-   - Visualize as dependency tree
-
-3. **Compile SQL**:
-   - Use `dbt_compile` to get rendered SQL
-   - Explain key transformations
-
-4. **Report**:
-   - Model purpose (from description)
-   - Materialization strategy
-   - Dependency graph
-   - Key SQL logic explained
+1. **Get model info**: Use `dbt_lineage` for metadata (description, tags, materialization)
+2. **Analyze dependencies**: Show upstream/downstream as tree
+3. **Compile SQL**: Use `dbt_compile` to get rendered SQL
+4. **Report**: Purpose, materialization, dependencies, key SQL logic
 
 ## Examples
 
@@ -48,9 +30,8 @@ Explain a dbt model's purpose, dependencies, and SQL logic.
 /explain fct_orders
 ```
 
-## Available Tools
+## Required MCP Tools
 
-Use these MCP tools:
 - `dbt_lineage` - Get model dependencies
 - `dbt_compile` - Get compiled SQL
 - `dbt_ls` - List related resources