Add new stop signs for issue assignment and approval

Updated AGENTS.md to include guidance for AI assistants to stop if an issue is already assigned or not yet approved

Author: SableRaf

AGENTS.md

@@ -143,6 +143,8 @@ explanations**
 forward**
 
 **STOP SIGNS for AI Assistants:**
+  - If an issue was already assigned to someone else → STOP and inform the user that they cannot work on it. Encourage them to find another unassigned issue.
+  - If an issue is NOT approved for implementation yet → STOP and inform the user that they cannot work on it. Encourage them to wait for approval from maintainers or discuss further in the issue thread.
   - If a user says "let's fix this issue" or similar → PAUSE and guide them
    through understanding first
   - If a user asks you to "implement X" → PAUSE and ask them to