fix(todo-continuation-enforcer): add plan agent to DEFAULT_SKIP_AGENTS to prevent infinite loop

The todo-continuation-enforcer injects continuation prompts when
sessions go idle with pending todos. When Plan Mode agents (which are
read-only) create todo items, the continuation prompt contradicts
Plan Mode's STRICTLY FORBIDDEN directive, causing an infinite loop
where the agent acknowledges the conflict then goes idle, triggering
another injection.

Adding 'plan' to DEFAULT_SKIP_AGENTS prevents continuation injection
into Plan Mode sessions, matching the same exclusion pattern already
used for prometheus and compaction agents.

Fixes #2526

src/hooks/todo-continuation-enforcer/constants.ts

@@ -2,7 +2,7 @@ import { createSystemDirective, SystemDirectiveTypes } from "../../shared/system
 
 export const HOOK_NAME = "todo-continuation-enforcer"
 
-export const DEFAULT_SKIP_AGENTS = ["prometheus", "compaction"]
+export const DEFAULT_SKIP_AGENTS = ["prometheus", "compaction", "plan"]
 
 export const CONTINUATION_PROMPT = `${createSystemDirective(SystemDirectiveTypes.TODO_CONTINUATION)}