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/plugin-discovery.md

@@ -0,0 +1,42 @@
+# Skill: Plugin Discovery
+
+Discover plugins in a marketplace by scanning for `.claude-plugin/` markers.
+
+## Discovery Process
+
+1. **Identify marketplace root**:
+   - Use provided path or default to current project root
+   - Look for `plugins/` subdirectory
+
+2. **Scan for plugins**:
+   - Find all directories containing `.claude-plugin/plugin.json`
+   - Each is a valid plugin
+
+3. **Build plugin list**:
+   - Extract plugin name from directory name
+   - Record path to plugin root
+
+## Standard Paths
+
+| Context | Path |
+|---------|------|
+| Installed | `~/.claude/plugins/marketplaces/leo-claude-mktplace/plugins/` |
+| Source | `~/claude-plugins-work/plugins/` |
+
+## Expected Structure
+
+```
+plugins/
+  plugin-name/
+    .claude-plugin/
+      plugin.json       # Required marker
+    commands/           # Command definitions
+    agents/             # Agent definitions (optional)
+    hooks/              # Hook definitions (optional)
+    skills/             # Skill files (optional)
+    README.md           # Interface documentation
+```
+
+## MCP Tool
+
+Use `parse_plugin_interface` with each discovered plugin's README.md path.