Expose backend identity and capacity errors

docs/cli.md

@@ -150,9 +150,12 @@ CLI inputs are file references. HTTP inline inputs are defined by the
 ## Output And Exit Behavior
 
 - `run` writes generated content to stdout, or to `--out` when supplied, and
-  writes a concise summary to stderr.
+  writes a concise summary to stderr. The summary includes `backend=<id>` when
+  Promptkit selected a backend; endpoint-only profiles omit it.
 - `render` writes prepared-run output to stdout, or to `--out` when supplied,
-  without a success summary.
+  without a success summary. Text output includes `selected_backend_id` after
+  `selected_profile_id` when Promptkit selected one; endpoint-only profiles
+  omit it.
 - `serve` writes startup and server errors to stderr.
 
 Exit statuses:
@@ -163,6 +166,10 @@ Exit statuses:
 | `1` | Parse, configuration, loading, rendering, generation, output-write, or other runtime error. |
 | `2` | `run` generated and wrote output, but validation failed. |
 
+A backend admission rejection is a runtime error and prints `run error: model
+backend capacity is exhausted`. The HTTP capacity response is defined in the
+[HTTP API reference](api.md).
+
 ## Workflows And Examples
 
 The [maintained render script](../examples/render-markdown-summary.sh) is a