Replace platform-specific 'which'/'where' commands with cross-platform Bun.which() API to fix Windows compatibility issues and simplify code.
Fixes:
- #1027: Comment-checker binary crashes on Windows (missing 'check' subcommand)
- #1036: Session-notification listens to non-existent events
- #1033: Infinite loop in session notifications
- #599: Doctor incorrectly reports OpenCode as not installed on Windows
- #1005: PowerShell path detection corruption on Windows
Changes:
- Use Bun.which() instead of spawning 'which'/'where' commands
- Add 'check' subcommand to comment-checker invocation
- Remove non-existent event listeners (session.updated, message.created)
- Prevent notification commands from resetting their own state
- Fix edge case: clear notifiedSessions if activity occurs during notification
All changes are cross-platform compatible and tested on Windows/Linux/macOS.
