fix(arcade): give ncurses games a TERM so they launch

The 80s arcade classics (Space Invaders/nInvaders, Pac-Man/pacman4console,
Tetris/tint, Moon Patrol/moon-buggy) are ncurses programs: initscr() fails
with "Error opening terminal" when TERM is unset. Game subprocesses were
built with exec.Command and no Env, so they inherited the agentbbs systemd
daemon's environment — which has no TERM — and every game exited before
drawing a frame. DOOM was unaffected because doom-ascii writes ANSI directly
and never touches terminfo.

Thread the client PTY's TERM through plugin.Context and hand each sandboxed
game a curated environment (TERM, PATH, HOME, LANG=C.UTF-8) instead of the
daemon's. Curating the env also stops leaking operator secrets (e.g.
COINPAY_API_KEY) into third-party game binaries.

Verified live on bbs.profullstack.com: Space Invaders and Pac-Man now render;
previously all four died instantly.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/plugin/plugin.go

@@ -25,6 +25,11 @@ type Context struct {
 	// Host is the BBS hostname (e.g. bbs.profullstack.com), for building
 	// member homepage URLs (https://Host/~name) and similar links.
 	Host string
+	// Term is the client PTY's terminal type (e.g. xterm-256color). Needed by
+	// sandboxed ncurses games (Space Invaders, Pac-Man, Tetris, Moon Patrol),
+	// which call initscr() and fail with "Error opening terminal" if TERM is
+	// unset — the systemd daemon environment has no TERM to inherit.
+	Term string
 }
 
 // Plugin is the only integration point between a feature and the hub.