Add research doc on interrupted shutdown dialog

[KTSCode]

Summary

• Add docs/research/2026-06-04-interrupted-shutdown-dialog.md cataloging how to get past the macOS " interrupted shutdown" dialog.
• Documents the root cause (non-root processes cannot force a shutdown past an app's applicationShouldTerminate: veto) and the current implementation (graceful System Events shutdown + 60s retry loop that just re-triggers the dialog).
• Catalogs four approaches with tradeoffs — passwordless sudo /sbin/shutdown, force-quit apps then graceful shutdown, loginwindow Apple events, pmset schedule — plus a security/governance note on the sudoers path.
• Research only, no code change. Picking A vs B is deferred to a follow-up spec.

Test plan

• mix quality (full suite, dialyzer included) passes via pre-commit hook — doc-only change, no Elixir touched