chore: remove compatibility field from various agent rules for consistency

.agents/rules/ai-prompt-engineering.md

@@ -1,7 +1,6 @@
 ---
 name: ai-prompt-engineering
 description: Create self-contained prompts for another AI assistant that lacks access to this workspace memory.
-compatibility: opencode
 trigger: always_on
 glob: ""
 ---
@@ -25,4 +24,3 @@ Use this skill when the user wants a prompt for another AI assistant, coding age
 - Prefer clear sections over long prose.
 - Include work-item ID and title when relevant.
 - Make assumptions explicit.
-