Set default value for workspace.root and updated config documentation

docs/config.md

@@ -66,7 +66,8 @@ whisperx:
 
 Why this is sufficient:
 
-- `workspace.root` and `whisperx.transcribe_url` are the core required pipeline fields.
+- `whisperx.transcribe_url` is required.
+- `workspace.root` is optional and defaults to `/var/lib/narratio`.
 - Seriatim and Audita sections may be omitted; defaults are applied.
 - Archive, storage, spool, normalize, and other optional sections get defaults when omitted.
 
@@ -135,7 +136,7 @@ Defaults listed here are effective runtime defaults after load.
 
 | Path | Type | Required | Default |
 | --- | --- | --- | --- |
-| `pipeline.workspace.root` | string | Yes | none |
+| `pipeline.workspace.root` | string | No | `/var/lib/narratio` |
 | `pipeline.workspace.cleanup_after_archive` | bool | No | `false` |
 | `pipeline.secrets.env_dir` | string | Conditional | none |
 | `pipeline.storage.backend` | string | No | empty |