Set TERM via on-remote infocmp probe, not env propagation

[dakra]

Summary

Fixes #224 (again). The previous fix (6a6101a) rebound tramp-terminal-type, but that variable is only consulted by tramp-handle-make-process (the generic handler). The ssh-method path most users hit (tramp-sh-handle-make-process) ignores it entirely. Plus, when the local default-toplevel process-environment already had TERM=xterm-ghostty (e.g. Emacs launched from ghostty), tramp-local-environment-variable-p stripped ghostel's pushed TERM as "ambient" — and the remote shell inherited TERM=dumb from TRAMP's connection shell, disabling readline/ZLE/fish line editing.

The new approach moves the TERM decision into the per-spawn /bin/sh -c wrapper so the remote shell decides after env propagation:

• Probe via infocmp xterm-ghostty. On hit, advertise ghostty (TERM/TERM_PROGRAM/TERM_PROGRAM_VERSION); on miss, fall back to xterm-256color so echo works.
• When ~/.local/share/ghostel/terminfo holds the bundled entry, prepend it to TERMINFO_DIRS before the probe — manual setups can co-locate terminfo with the shell scripts (no tic, no ~/.terminfo).
• The local on-disk TERMINFO path is no longer pushed to the remote (it pointed at a directory the remote couldn't read and per terminfo(5) was suppressing system lookups).
• tramp-terminal-type let-binding from 6a6101a is removed.

README "Option 2: Manual setup" updated with the one-shot install recipe (shell scripts + co-located terminfo in one mkdir + scp burst).

Test plan

• make -j4 all green (219/219 elisp, 47/47 native, lint, build)
• Wrapper-script POSIX portability (dash, bash, zsh, ksh)
• TRAMP-quoting roundtrip verified — backslash-escaped tildes survive the connection-shell strip step, inner /bin/sh -c sees literal ~/... and expands correctly
• Convention-path detection verified end-to-end via HOME override + tramp-shell-quote-argument simulation
• New ghostel-test-remote-term-preamble and ghostel-test-spawn-pty-uses-remote-term-preamble cover the wrapper shape, the env-doesn't-leak-TERM-or-TERMINFO contract, and the load-bearing order (TERMINFO_DIRS prepend BEFORE the infocmp probe)
• Live verification on a real TRAMP /ssh: session (manual, since TRAMP isn't covered by the automated test suite)

Behavioral matrix

Setup	Before	After
No integration, nothing on remote	TERM=dumb → no echo	TERM=xterm-256color → echo works
ghostel-tramp-shell-integration t	works	unchanged (TERMINFO env reaches the probe)
Manual install via ~/.terminfo	broken (bogus local TERMINFO suppressed lookup)	works (probe finds it)
Manual install via co-located ~/.local/share/ghostel/terminfo	n/a	works (newly supported)