docs: add unreferenced command behavior note to quick reference

Users who define commands but forget to add {{ commands.<name> }}
placeholders won't see the output in their prompt. This detail was
documented in how-it-works.md (48e2ce6) but missing from the
quick-reference page that users bookmark.

Co-authored-by: Ralphify <noreply@ralphify.co>

Author: 2 people

docs/quick-reference.md

@@ -95,6 +95,7 @@ Your instructions here. Use {{ args.dir }} for user arguments.
 ```
 
 - Output includes stdout + stderr regardless of exit code
+- Only commands referenced by a placeholder appear in the prompt — unreferenced commands still run but their output is excluded
 - Unmatched placeholders resolve to empty string
 - Must be `commands` (plural)