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/visual-output.md

@@ -0,0 +1,88 @@
+# Skill: Visual Output
+
+Standard visual formatting for contract-validator commands.
+
+## Command Header
+
+Display at the start of every command:
+
+```
++----------------------------------------------------------------------+
+|  CONTRACT-VALIDATOR - [Command Name]                                 |
++----------------------------------------------------------------------+
+```
+
+## Result Boxes
+
+### Success Box
+```
++====================================================================+
+|          CONTRACT-VALIDATOR [ACTION] COMPLETE                       |
++====================================================================+
+| Item 1:          [checkmark] Status                                 |
+| Item 2:          [checkmark] Status                                 |
++====================================================================+
+```
+
+### Error Box
+```
++====================================================================+
+|          CONTRACT-VALIDATOR [ACTION] FAILED                         |
++====================================================================+
+| Error:           [description]                                      |
+| Suggestion:      [fix]                                              |
++====================================================================+
+```
+
+## Summary Tables
+
+### Plugin Summary
+```
+| Plugin      | Commands | Agents | Tools |
+|-------------|----------|--------|-------|
+| projman     | 12       | 4      | 26    |
+| data-platform| 7       | 2      | 32    |
+```
+
+### Issue Summary
+```
+| Severity | Count |
+|----------|-------|
+| ERROR    | 2     |
+| WARNING  | 5     |
+| INFO     | 8     |
+```
+
+## Mermaid Diagrams
+
+For dependency graphs, use flowchart TD format:
+
+```mermaid
+flowchart TD
+    subgraph group_name["Group Label"]
+        node1["Plugin Name"]
+    end
+
+    node1 -->|"label"| node2
+    node1 -.->|"optional"| node3
+
+    classDef required stroke:#e74c3c,stroke-width:2px
+    classDef optional stroke:#f39c12,stroke-dasharray:5 5
+```
+
+## Text Format Alternative
+
+For non-graphical output:
+
+```
+DEPENDENCY GRAPH
+================
+
+Plugins: 12
+MCP Servers: 4
+Dependencies: 8 (5 required, 3 optional)
+
+MCP Server Groups:
+  gitea: projman, pr-review
+  data-platform: data-platform
+```