docs: add supported runners table to frontmatter reference (#17361)

Author: Mossaka

docs/src/content/docs/reference/frontmatter.md

@@ -450,6 +450,16 @@ runs-on: ubuntu-latest               # Defaults to ubuntu-latest (main job only)
 timeout-minutes: 30                  # Defaults to 20 minutes
 ```
 
+**Supported runners for `runs-on:`**
+
+| Runner | Status |
+|--------|--------|
+| `ubuntu-latest` | ✅ Default. Recommended for most workflows. |
+| `ubuntu-24.04` / `ubuntu-22.04` | ✅ Supported. |
+| `ubuntu-24.04-arm` | ✅ Supported. Linux ARM64 runner. |
+| `macos-*` | ❌ Not supported. Docker is unavailable on macOS runners (no nested virtualization). See [FAQ](/gh-aw/reference/faq/). |
+| `windows-*` | ❌ Not supported. AWF requires Linux. |
+
 ### Workflow Concurrency Control (`concurrency:`)
 
 Automatically generates concurrency policies for the agent job. See [Concurrency Control](/gh-aw/reference/concurrency/).