Documented Agent label governance

Property	Value
description	Recorded the Linear rule that only Agent-labeled issues are executable by Codex or other AI agents.
version	0.5.3
type	Update
related	MAC

• Recorded that Agent is the execution label for Codex and AI-agent work.
• Clarified that issues without Agent are human-only unless explicitly re-authorized.
• Added status and comment hygiene expectations for Linear issue execution.