The check_opencode_session_exists() function was fundamentally broken
because 'opencode session list' does not include forked sessions,
regardless of output format (table or JSON). This caused false
'session expired' reports even when sessions were fully functional.
Changes:
- Remove session check from cmd_status() - now returns 'ok' if session registered
- Remove session check from cmd_delegate() - let opencode run fail naturally
- Remove warning from cmd_continue() - proceed regardless
- Simplify cmd_doctor() - just show registered sessions
- Update test to reflect new behavior
Issue #38: #38
