fix(cli-run): prevent premature exit when background tasks are active

When using `opencode run`, the process exits prematurely if the main
agent dispatches background subtasks. The CLI completion checker relies
on `session.children()` + `session.status()` APIs which cannot see
tasks in "pending" state (no session created yet) or tasks whose
sessions are momentarily idle between operations.

This fix bridges BackgroundManager state to the CLI completion checker
using the existing run-continuation-state marker system:

- Add "background-task" continuation marker source
- BackgroundManager writes/clears markers on task lifecycle events
  (launch, cancel, complete, crash)
- CLI completion checker blocks exit when marker is active
- Fix todo-continuation-enforcer to also check "pending" task status

Closes #3452

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

@@ -79,7 +79,7 @@ export async function injectContinuation(args: {
   }
 
   const hasRunningBgTasks = backgroundManager
-    ? backgroundManager.getTasksByParentSession(sessionID).some((task: { status: string }) => task.status === "running")
+    ? backgroundManager.getTasksByParentSession(sessionID).some((task: { status: string }) => task.status === "running" || task.status === "pending")
     : false
 
   if (hasRunningBgTasks) {

src/hooks/todo-continuation-enforcer/idle-event.ts

@@ -71,7 +71,7 @@ export async function handleSessionIdle(args: {
   }
 
   const hasRunningBgTasks = backgroundManager
-    ? backgroundManager.getTasksByParentSession(sessionID).some((task: { status: string }) => task.status === "running")
+    ? backgroundManager.getTasksByParentSession(sessionID).some((task: { status: string }) => task.status === "running" || task.status === "pending")
     : false
 
   if (hasRunningBgTasks) {