Upgrade Sentry: tracing, breadcrumbs, dSYM upload (#366)

* Upgrade Sentry: tracing, breadcrumbs, dSYM upload

- Enhanced Sentry SDK init with performance tracing (10% sample),
  explicit app hang timeout, stack trace attachment, and HTTP
  failure capture
- Added breadcrumbs for key user actions: workspace switch/create/close,
  split creation, command palette open/close, app focus — these give
  context to hang/crash reports
- Added dSYM upload step to nightly and release CI workflows so hang
  stacks are fully symbolicated (requires SENTRY_AUTH_TOKEN secret)
- Created SentryHelper.swift with lightweight breadcrumb helper

Closes https://github.com/manaflow-ai/cmux/issues/365

* Remove command palette breadcrumbs

Not useful for hang diagnosis — keep only workspace/tab/split/focus
breadcrumbs that correlate with heavy operations.

Sources/SentryHelper.swift

@@ -0,0 +1,9 @@
+import Sentry
+
+/// Add a Sentry breadcrumb for user-action context in hang/crash reports.
+func sentryBreadcrumb(_ message: String, category: String = "ui", data: [String: Any]? = nil) {
+    let crumb = Breadcrumb(level: .info, category: category)
+    crumb.message = message
+    crumb.data = data
+    SentrySDK.addBreadcrumb(crumb)
+}