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/skills/visual-header.md

@@ -0,0 +1,45 @@
+# Visual Header
+
+## Standard Format
+
+Display at the start of every command execution:
+
+```
++----------------------------------------------------------------------+
+|  DATA-PLATFORM - [Command Name]                                      |
++----------------------------------------------------------------------+
+```
+
+## Command Headers
+
+| Command | Header Text |
+|---------|-------------|
+| initial-setup | Setup Wizard |
+| ingest | Ingest |
+| profile | Data Profile |
+| schema | Schema Explorer |
+| data-quality | Data Quality |
+| run | dbt Run |
+| dbt-test | dbt Tests |
+| lineage | Lineage |
+| lineage-viz | Lineage Visualization |
+| explain | Model Explanation |
+
+## Summary Box Format
+
+For completion summaries:
+
+```
++============================================================+
+|            DATA-PLATFORM [OPERATION] COMPLETE              |
++============================================================+
+| Component:         [Status]                                |
+| Component:         [Status]                                |
++============================================================+
+```
+
+## Status Indicators
+
+- Success: `[check]` or `Ready`
+- Warning: `[!]` or `Partial`
+- Failure: `[X]` or `Failed`