feat(notification): add cmux as notification provider

Add cmux as the highest-priority notification provider on macOS.
cmux delivers notifications via native UNUserNotificationCenter,
properly attributed to the cmux app instead of Script Editor.

Notification priority: cmux > terminal-notifier > osascript

Tests cover the full fallback chain:
- cmux available: uses cmux, skips others
- cmux fails: falls back to terminal-notifier
- cmux + terminal-notifier fail: falls back to osascript
- cmux not available: skips to terminal-notifier

Fixes #3628

src/hooks/session-notification-utils.ts

@@ -50,9 +50,13 @@ export const getAfplayPath = createCommandFinder("afplay")
 export const getPaplayPath = createCommandFinder("paplay")
 export const getAplayPath = createCommandFinder("aplay")
 export const getTerminalNotifierPath = createCommandFinder("terminal-notifier")
+export const getCmuxPath = createCommandFinder("cmux")
 
 export function startBackgroundCheck(platform: Platform): void {
   if (platform === "darwin") {
+    getCmuxPath().catch((error) => {
+      logBackgroundCheckError("cmux", error)
+    })
     getOsascriptPath().catch((error) => {
       logBackgroundCheckError("osascript", error)
     })