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/cli/run/completion.ts

@@ -20,6 +20,11 @@ export async function checkCompletionConditions(ctx: RunContext): Promise<boolea
       return false
     }
 
+    if (continuationState.hasActiveBackgroundTaskMarker) {
+      logWaiting(ctx, continuationState.activeHookMarkerReason ?? "background tasks are active")
+      return false
+    }
+
     if (!await areAllChildrenIdle(ctx)) {
       return false
     }

src/cli/run/continuation-state.ts

@@ -16,6 +16,7 @@ export interface ContinuationState {
   hasActiveRalphLoop: boolean
   hasHookMarker: boolean
   hasTodoHookMarker: boolean
+  hasActiveBackgroundTaskMarker: boolean
   hasActiveHookMarker: boolean
   activeHookMarkerReason: string | null
 }
@@ -32,6 +33,7 @@ export async function getContinuationState(
     hasActiveRalphLoop: hasActiveRalphLoopContinuation(directory, sessionID),
     hasHookMarker: marker !== null,
     hasTodoHookMarker: marker?.sources.todo !== undefined,
+    hasActiveBackgroundTaskMarker: marker?.sources["background-task"]?.state === "active",
     hasActiveHookMarker: isContinuationMarkerActive(marker),
     activeHookMarkerReason: getActiveContinuationMarkerReason(marker),
   }