alacritty terminal adapter

[mduvarci12]

Summary

Adds a new terminal backend for Alacritty, following the
launch-only adapter contract already used by windows-terminal.sh.

Alacritty has no AppleScript dictionary and no built-in way to return a stable,
queryable window/tab id, so this adapter can open a terminal surface per
role but cannot reliably track or reopen it afterward. Concretely:

• terminal_backend_can_open_sessions → return 0 (it can open sessions)
• terminal_backend_tracks_windows → return 1 (no stable id to track)

This means SwarmForge opens one Alacritty window per configured role and skips
the watchdog for those windows, exactly like the existing Windows Terminal
adapter. Closing the first (cleanup) window still works normally, since that
path doesn't depend on window tracking.

Usage

SWARMFORGE_TERMINAL=alacritty ./swarm

What's implemented

• terminal_backend_label — returns "Alacritty"
• terminal_backend_can_open_sessions — return 0
• terminal_backend_tracks_windows — return 1 (launch-only)
• terminal_open_session — opens a new Alacritty window via open -na Alacritty
  that attaches directly to the assigned tmux socket/session
• terminal_window_exists / terminal_close_window — no-ops consistent with
  the launch-only contract (nothing SwarmForge can reliably track or close)

Testing

Tested manually on macOS (Sonoma) with Alacritty installed via Homebrew:

• SWARMFORGE_TERMINAL=alacritty ./swarm opens one Alacritty window per role
• Each window correctly attaches to its assigned tmux session
• Cleanup (closing the first configured window) still tears down the swarm as expected

Limitations / follow-ups

• No watchdog / auto-reopen if a role's Alacritty window is closed manually,
  same limitation as the Windows Terminal adapter.
• A future version could attempt id tracking via System Events window
  matching (by title), but this felt more fragile than useful for a first pass —
  happy to discuss if maintainers want that instead.