Add nushell shell integration

[dakra]

Summary

Adds nushell (nu) as a first-class supported shell alongside bash, zsh, and
fish — for auto-injection, manual source, and remote TRAMP sessions.

Design

Unlike the other shells, nushell emits OSC 7 (cwd), OSC 133 (prompt marks),
and OSC 2 (title) natively via its built-in $env.config.shell_integration
(on by default), and derives OSC 7's host from gethostname(2) — so it already
matches Emacs (system-name) the way the bash/zsh capture-once logic does,
without the $HOSTNAME-pollution workarounds. So etc/shell/ghostel.nu only
adds the ghostel-specific pieces:

• ghostel_cmd — the OSC 52 ;e bridge for calling Elisp from the shell.
• The outbound ssh terminfo-install wrapper (nushell port of the bash/fish
  install-and-cache logic, runtime-gated on GHOSTEL_SSH_INSTALL_TERMINFO).

Injection mirrors fish: nushell vendor-autoloads from XDG_DATA_DIRS
(<dir>/nushell/vendor/autoload/), so 'nu reuses fish's bootstrap dir and
GHOSTEL_SHELL_INTEGRATION_XDG_DIR cleanup. The autoload shim locates
ghostel.nu via path self because nushell's source needs a
parse-time-constant path. Remote/TRAMP uses --execute "source …" (the
analogue of fish's -C), and nushell starts login+interactive (-l -i).

Changes

• New: etc/shell/ghostel.nu and
  etc/shell/bootstrap/nushell/vendor/autoload/integration.nu.
• lisp/ghostel.el: shell detection (nu/nushell → 'nu), local injection
  (folded into the fish XDG branch), remote TRAMP branch, default remote args,
  and the ghostel-shell-integration / ghostel-tramp-shell-integration
  customizations.
• Tests: shell detection, default remote args, the auto-inject chain (drives
  nu --execute, asserting the autoload → path self → source chain and XDG
  cleanup), and ghostel_cmd's OSC 52 output.
• Docs: README (manual setup, local + remote) and CHANGELOG.

Caveat

Directory tracking and prompt navigation rely on nushell's native
shell_integration.osc7/.osc133 (enabled by default); disabling them in your
nushell config disables those features inside ghostel. Documented in the README.

Testing

• make -j8 all green (build + lint + all tests, 0 unexpected).
• Live-verified in a sandboxed Emacs: a real ghostel buffer running nushell —
  integration loaded (ghostel_cmd/ssh defined), cd /tmp updates
  default-directory, and XDG_DATA_DIRS is cleaned with no env leak.