fix(session-notification): use node:child_process to avoid Bun shell GC crash (#543)

Replace Bun shell template literals (ctx.$) with node:child_process.spawn
to work around Bun's ShellInterpreter garbage collection bug on Windows.

This bug causes segmentation faults in deinitFromFinalizer during heap
sweeping when shell operations are used repeatedly over time.

Bug references:
- oven-sh/bun#23177 (closed incomplete)
- oven-sh/bun#24368 (still open)
- Pending fix: oven-sh/bun#24093

The fix applies to all platforms for consistency and safety.

src/hooks/session-notification.test.ts

@@ -1,4 +1,6 @@
-import { describe, expect, test, beforeEach, afterEach, spyOn } from "bun:test"
+import { describe, expect, test, beforeEach, afterEach, spyOn, mock } from "bun:test"
+import { EventEmitter } from "node:events"
+import * as childProcess from "node:child_process"
 
 import { createSessionNotification } from "./session-notification"
 import { setMainSession, subagentSessions } from "../features/claude-code-session-state"
@@ -6,20 +8,11 @@ import * as utils from "./session-notification-utils"
 
 describe("session-notification", () => {
   let notificationCalls: string[]
+  let spawnMock: ReturnType<typeof spyOn>
 
   function createMockPluginInput() {
     return {
-      $: async (cmd: TemplateStringsArray | string, ...values: any[]) => {
-        // #given - track notification commands (osascript, notify-send, powershell)
-        const cmdStr = typeof cmd === "string" 
-          ? cmd 
-          : cmd.reduce((acc, part, i) => acc + part + (values[i] ?? ""), "")
-        
-        if (cmdStr.includes("osascript") || cmdStr.includes("notify-send") || cmdStr.includes("powershell")) {
-          notificationCalls.push(cmdStr)
-        }
-        return { stdout: "", stderr: "", exitCode: 0 }
-      },
+      $: async () => ({ stdout: "", stderr: "", exitCode: 0 }),
       client: {
         session: {
           todo: async () => ({ data: [] }),
@@ -32,6 +25,18 @@ describe("session-notification", () => {
   beforeEach(() => {
     notificationCalls = []
     
+    // Mock spawn to track notification commands
+    // Uses node:child_process.spawn instead of Bun shell to avoid GC crash
+    spawnMock = spyOn(childProcess, "spawn").mockImplementation((cmd: string, args?: string[]) => {
+      // Track notification commands (osascript, notify-send, powershell)
+      if (cmd.includes("osascript") || cmd.includes("notify-send") || cmd.includes("powershell")) {
+        notificationCalls.push(`${cmd} ${(args ?? []).join(" ")}`)
+      }
+      const emitter = new EventEmitter()
+      setTimeout(() => emitter.emit("close", 0), 0)
+      return emitter as any
+    })
+    
     spyOn(utils, "getOsascriptPath").mockResolvedValue("/usr/bin/osascript")
     spyOn(utils, "getNotifySendPath").mockResolvedValue("/usr/bin/notify-send")
     spyOn(utils, "getPowershellPath").mockResolvedValue("powershell")