Add research doc on Do Not Disturb control

[KTSCode]

Summary

• Add docs/research/2026-06-04-do-not-disturb-control.md cataloging ways for Severance to turn macOS Do Not Disturb / Focus on and off
• Compares approaches: Shortcuts CLI (shortcuts run, recommended), poking ~/Library/DoNotDisturb/DB/Assertions.json + nudging donotdisturbd, AppleScript UI scripting, the dead legacy defaults write, and third-party CLIs/libs
• Flags that the Assertions.json path already broke on macOS 26 (Tahoe), the OS this machine runs — so the file-poke approaches are version-fragile and the Shortcuts path is the only documented/sanctioned one
• Covers the two user asks: a config-fn/script run at the DND on/off boundaries (modeled on the existing publisher contract) and a calendar guard (Shortcuts "Get Upcoming Events" / icalBuddy / EventKit) to avoid silencing real meetings
• Grounds the integration shapes in real seams (Severance.System callbacks, the publisher worker, config.exs eval) and lists open questions for the follow-up spec

Test plan

• mix quality (full suite incl. dialyzer) green via pre-commit hook
• Doc-only change — no application code, no new tests