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/dbt-workflow.md

@@ -0,0 +1,85 @@
+# dbt Workflow
+
+## Pre-Validation (MANDATORY)
+
+**Always run `dbt_parse` before any dbt operation.**
+
+This validates:
+- dbt_project.yml syntax
+- Model SQL syntax
+- schema.yml definitions
+- Deprecated syntax (dbt 1.9+)
+
+If validation fails, show errors and STOP.
+
+## Model Selection Syntax
+
+| Pattern | Meaning |
+|---------|---------|
+| `model_name` | Single model |
+| `+model_name` | Model and upstream dependencies |
+| `model_name+` | Model and downstream dependents |
+| `+model_name+` | Model with all dependencies |
+| `tag:name` | Models with specific tag |
+| `path:models/staging` | Models in path |
+| `test_type:schema` | Schema tests only |
+| `test_type:data` | Data tests only |
+
+## Execution Workflow
+
+1. **Parse**: `dbt_parse` - Validate project
+2. **Run**: `dbt_run` - Execute models
+3. **Test**: `dbt_test` - Run tests
+4. **Build**: `dbt_build` - Run + test together
+
+## Test Types
+
+### Schema Tests
+Defined in `schema.yml`:
+- `unique` - No duplicate values
+- `not_null` - No null values
+- `accepted_values` - Value in allowed list
+- `relationships` - Foreign key integrity
+
+### Data Tests
+Custom SQL in `tests/` directory:
+- Return rows that fail assertion
+- Zero rows = pass, any rows = fail
+
+## Materialization Types
+
+| Type | Description |
+|------|-------------|
+| `view` | Virtual table, always fresh |
+| `table` | Physical table, full rebuild |
+| `incremental` | Append/merge new rows only |
+| `ephemeral` | CTE, no physical object |
+
+## Exit Codes
+
+| Code | Meaning |
+|------|---------|
+| 0 | Success |
+| 1 | Test/run failure |
+| 2 | dbt error (parse failure) |
+
+## Result Formatting
+
+```
+=== dbt [Operation] Results ===
+Project: [project_name]
+Selection: [selection_pattern]
+
+--- Summary ---
+Total: X models/tests
+PASS:  X (%)
+FAIL:  X (%)
+WARN:  X (%)
+SKIP:  X (%)
+
+--- Details ---
+[Model/Test details with status]
+
+--- Failure Details ---
+[Error messages and remediation]
+```