P2.3: confirm legacy autostart tests are deleted

The legacy tests TestParseRegistryRunValue (Windows registry-based
autostart) and TestLinuxAutostartRemovesLegacyDesktopEntry (Linux systemd
cleanup) were already removed in P2.1 and P2.2. All remaining tests in
autostart_windows_test.go and autostart_linux_test.go test the current,
non-legacy implementation (Startup folder shortcuts and desktop files).

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/PRE-RELEASE-TASKS.md

@@ -90,7 +90,7 @@ These land together because both edit `domain/job.go` and `storage/store.go`.
 ### Phase 2 — PySentry legacy removal
 - [x] P2.1 — Windows autostart legacy code
 - [x] P2.2 — Linux autostart legacy code
-- [ ] P2.3 — Delete legacy autostart tests
+- [x] P2.3 — Delete legacy autostart tests
 - [ ] P2.4 — `.gitignore` / `.dockerignore`
 
 ### Phase 3 — Task-queue model + settings