docs: add /review user documentation

Add comprehensive user documentation for the /review command covering:
- Quick start examples for all modes (local, PR, file, --comment)
- Pipeline overview with all steps explained
- Review agents table (5 agents + their focus areas)
- Deterministic analysis (supported languages and tools)
- Severity levels and PR comment filtering rules
- Autofix workflow
- PR inline comments (what gets posted vs terminal-only)
- Follow-up actions (fix/post comments/commit)
- Project review rules (.qwen/review-rules.md etc.)
- Incremental review and caching
- Review report persistence
- Cross-file impact analysis
- Design philosophy

Also add /review and /simplify to the commands reference page
under a new "Built-in Skills" section with link to full docs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/users/features/commands.md

@@ -71,7 +71,18 @@ Commands for managing AI tools and models.
 | `/extensions`    | List all active extensions in current session     | `/extensions`                                 |
 | `/memory`        | Manage AI's instruction context                   | `/memory add Important Info`                  |
 
-### 1.5 Side Question (`/btw`)
+### 1.5 Built-in Skills
+
+These commands invoke bundled skills that provide specialized workflows.
+
+| Command     | Description                                                         | Usage Examples                                    |
+| ----------- | ------------------------------------------------------------------- | ------------------------------------------------- |
+| `/review`   | Review code changes with 5 parallel agents + deterministic analysis | `/review`, `/review 123`, `/review 123 --comment` |
+| `/simplify` | Review changed code for reuse, quality, and efficiency              | `/simplify`                                       |
+
+See [Code Review](./code-review.md) for full `/review` documentation.
+
+### 1.6 Side Question (`/btw`)
 
 The `/btw` command allows you to ask quick side questions without interrupting or affecting the main conversation flow.