Currently the init phase will catch a broken build but not a nonexistent namespace. If the user typos the namespace (e.g., 'Omni/Aget/Coder' instead of 'Omni/Agent/Coder'), the coder will fail with a confusing build error.
Add validation in runCoderInit: 1. Check if the namespace file exists (for .hs, .py, etc.) 2. For directory namespaces, check the directory exists 3. Return a clear InitFailed error if namespace is invalid
This gives immediate, clear feedback instead of a cryptic build failure.