test(cli): add comprehensive clipboard tests for SSH-aware copy implementation

- Test message subscription system (show, clear, timer cancellation)
- Test empty/whitespace text handling
- Test success message formatting and truncation
- Test error handling when both copy methods fail
- Test SSH session detection behavior (SSH_CLIENT, SSH_TTY, SSH_CONNECTION)
- Test OSC52 behavior (TERM=dumb, 32KB size limit, tmux/screen passthrough)
- Integration test for actual clipboard copy on macOS

Fixed misleading header comment to accurately describe test coverage.
Covers the SSH-aware clipboard fix that uses OSC52 first for remote
sessions and platform tools first for local sessions.

Author: brandonkachen