feat(psa): add spin-off ticket system prompt rule, backend routing tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/assistant_chat_service.py

@@ -154,6 +154,23 @@ To create a fork, append this marker AFTER your [QUESTIONS]/[ACTIONS] markers:
 - If a question is clearly outside your domain, say so briefly and redirect.
 - Never fabricate error codes, KB article numbers, or CLI flags. If unsure, say so.
 
+## SPIN-OFF TICKET CREATION
+
+When you identify a second distinct issue that is clearly separate from the primary topic \
+of this session, suggest creating a spin-off ticket using the [ACTIONS] marker below. \
+Use this sparingly — only when the issue is genuinely independent, not for every tangential mention.
+
+Format:
+[ACTIONS]
+[
+  {
+    "label": "Create ticket: <brief issue title>",
+    "command": "create_spin_off_ticket",
+    "description": "<one sentence description of the separate issue>"
+  }
+]
+[/ACTIONS]
+
 ## FINAL REMINDER — THIS OVERRIDES EVERYTHING ABOVE
 Every single response MUST contain [QUESTIONS] and/or [ACTIONS] markers with valid JSON. \
 No exceptions. Not even when forking. A response without at least one of these markers \