fix: fall back TERM when terminfo is missing

[alohays]

Summary

• env.sh: generic TERM fallback via infocmp check — when the advertised terminal lacks a local terminfo entry (e.g. xterm-ghostty over SSH), automatically fall back to xterm-256color so tmux and other ncurses tools can start
• Ghostty config: enable ssh-terminfo and ssh-env shell integration features so Ghostty v1.2.0+ auto-installs its terminfo on remote hosts, with automatic xterm-256color fallback when blocked

Two-layer approach

Layer	Where	What
Ghostty config	Client (local machine)	Auto-install terminfo on SSH targets; fall back if blocked
env.sh fallback	Server (remote host)	Catch-all for any terminal with missing terminfo

Why both?

• Layer 1 (Ghostty) handles the common case and preserves full Ghostty features
• Layer 2 (env.sh) catches edge cases: non-Ghostty terminals, restricted SSH environments (AWS EC2, etc.), hosts where shell integration is blocked
• Complements PR fix: add terminfo capability guard to interactive.d scripts #27's dotfiles_term_capable() which guards ZLE plugins but doesn't fix TERM itself

Test plan

• source ~/.zshenv && echo $TERM → xterm-256color on host without ghostty terminfo
• tmux ls → no "missing or unsuitable terminal" error
• On Ghostty with shell integration: reconnect → terminfo auto-installed → TERM=xterm-ghostty preserved

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