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/lineage-analysis.md

@@ -0,0 +1,73 @@
+# Lineage Analysis
+
+## Lineage Workflow
+
+1. **Get lineage data** via `dbt_lineage`
+2. **Build dependency graph** (upstream + downstream)
+3. **Visualize** (ASCII tree or Mermaid)
+4. **Report** critical path and refresh implications
+
+## ASCII Tree Format
+
+```
+Sources:
+  |-- raw_customers (source)
+  |-- raw_orders (source)
+
+model_name (materialization)
+  |-- upstream:
+  |   |-- stg_model (view)
+  |       |-- raw_source (source)
+  |-- downstream:
+      |-- fct_model (incremental)
+      |-- rpt_model (table)
+```
+
+## Mermaid Diagram Format
+
+```mermaid
+flowchart LR
+    subgraph Sources
+        raw_data[(raw_data)]
+    end
+
+    subgraph Staging
+        stg_model[stg_model]
+    end
+
+    subgraph Marts
+        dim_model{{dim_model}}
+    end
+
+    raw_data --> stg_model
+    stg_model --> dim_model
+```
+
+## Mermaid Node Shapes
+
+| Materialization | Shape | Syntax |
+|-----------------|-------|--------|
+| source | Cylinder | `[(name)]` |
+| view | Rectangle | `[name]` |
+| table | Double braces | `{{name}}` |
+| incremental | Hexagon | `{{name}}` |
+| ephemeral | Dashed | `[/name/]` |
+
+## Mermaid Options
+
+| Flag | Description |
+|------|-------------|
+| `--direction TB` | Top-to-bottom (default: LR) |
+| `--depth N` | Limit lineage depth |
+
+## Styling Target Model
+
+```mermaid
+style target_model fill:#f96,stroke:#333,stroke-width:2px
+```
+
+## Usage Tips
+
+1. **Documentation**: Copy Mermaid to README.md
+2. **GitHub/GitLab**: Both render Mermaid natively
+3. **Live Editor**: https://mermaid.live for interactive editing