fix(background-agent): prevent zombie processes by aborting sessions on shutdown (#1240) (#1243)

- BackgroundManager.shutdown() now aborts all running child sessions via
  client.session.abort() before clearing state, preventing orphaned
  opencode processes when parent exits
- Add onShutdown callback to BackgroundManager constructor, used to
  trigger TmuxSessionManager.cleanup() on process exit signals
- Interactive bash session hook now aborts tracked subagent opencode
  sessions when killing tmux sessions (defense-in-depth)
- Add 4 tests verifying shutdown abort behavior and callback invocation

Closes #1240

src/hooks/interactive-bash-session/index.ts

@@ -6,6 +6,7 @@ import {
 } from "./storage";
 import { OMO_SESSION_PREFIX, buildSessionReminderMessage } from "./constants";
 import type { InteractiveBashSessionState } from "./types";
+import { subagentSessions } from "../../features/claude-code-session-state";
 
 interface ToolExecuteInput {
   tool: string;
@@ -146,7 +147,7 @@ function findSubcommand(tokens: string[]): string {
   return ""
 }
 
-export function createInteractiveBashSessionHook(_ctx: PluginInput) {
+export function createInteractiveBashSessionHook(ctx: PluginInput) {
   const sessionStates = new Map<string, InteractiveBashSessionState>();
 
   function getOrCreateState(sessionID: string): InteractiveBashSessionState {
@@ -178,6 +179,10 @@ export function createInteractiveBashSessionHook(_ctx: PluginInput) {
         await proc.exited;
       } catch {}
     }
+
+    for (const sessionId of subagentSessions) {
+      ctx.client.session.abort({ path: { id: sessionId } }).catch(() => {})
+    }
   }
 
   const toolExecuteAfter = async (