feat: add GET /api/v2/templatebuilder/bases endpoint (#26116)

Implement `GET /api/v2/templatebuilder/bases`, which returns the list of
base templates available in the template builder. Reads from the bundled
catalog by cross-referencing `templatebuilder.BaseTemplateIDs()` with
`examples.List()`, enriching each entry with the OS from the `exampleID
-> OS` map.

The endpoint is gated behind the template builder feature flag (returns
404 when disabled) and requires `policy.ActionRead` on
`rbac.ResourceTemplate`.

Depends on #26115

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.

docs/manifest.json

@@ -1611,6 +1611,10 @@
 							"title": "Tasks",
 							"path": "./reference/api/tasks.md"
 						},
+						{
+							"title": "TemplateBuilder",
+							"path": "./reference/api/templatebuilder.md"
+						},
 						{
 							"title": "Templates",
 							"path": "./reference/api/templates.md"