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/claude-config-maintainer/skills/visual-header.md

@@ -0,0 +1,52 @@
+# Visual Header Display
+
+This skill defines the standard visual header for claude-config-maintainer commands.
+
+## Header Format
+
+```
++-----------------------------------------------------------------+
+|  CONFIG-MAINTAINER - [Command Name]                              |
++-----------------------------------------------------------------+
+```
+
+## Command-Specific Headers
+
+### /config-analyze
+```
++-----------------------------------------------------------------+
+|  CONFIG-MAINTAINER - CLAUDE.md Analysis                          |
++-----------------------------------------------------------------+
+```
+
+### /config-optimize
+```
++-----------------------------------------------------------------+
+|  CONFIG-MAINTAINER - CLAUDE.md Optimization                      |
++-----------------------------------------------------------------+
+```
+
+### /config-lint
+```
++-----------------------------------------------------------------+
+|  CONFIG-MAINTAINER - CLAUDE.md Lint                              |
++-----------------------------------------------------------------+
+```
+
+### /config-diff
+```
++-----------------------------------------------------------------+
+|  CONFIG-MAINTAINER - CLAUDE.md Diff                              |
++-----------------------------------------------------------------+
+```
+
+### /config-init
+```
++-----------------------------------------------------------------+
+|  CONFIG-MAINTAINER - CLAUDE.md Initialization                    |
++-----------------------------------------------------------------+
+```
+
+## Usage
+
+Display the header at the start of command execution, before any analysis or output.