docs(agents): behaviour → behavior (Argus smoke test)

Single-word docs fix to exercise the newly-ported Argus AI PR review
workflow end-to-end. AGENTS.md is the Python SDK's API surface
reference for agents; US spelling matches the rest of the file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bokelley

AGENTS.md

@@ -99,7 +99,7 @@ else:
 
 ## Optional Protocol Extensions
 
-These protocols are opt-in upgrades for `DecisioningPlatform` subclasses. Implement them to unlock advanced framework behaviour.
+These protocols are opt-in upgrades for `DecisioningPlatform` subclasses. Implement them to unlock advanced framework behavior.
 
 | Protocol | Method | Domain | Description |
 |---|---|---|---|