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.