Phase 3 of MUL-2295. Adds `multica issue terminal <issue-id>` which dials
the Phase 2 /ws/issues/{id}/terminal endpoint, performs first-frame auth
with the existing PAT/JWT, and runs an interactive PTY through the
daemon-side terminal manager from Phase 1. SIGWINCH on unix /
poll on windows pushes resize frames; ssh-style `<enter>~.` detaches.
Co-authored-by: multica-agent <github@multica.ai>
