feat(termio): port pty, child command spawning, shell integration

Ports the parts of Ghostty's IO layer that don't depend on the terminal:
- pty.zig: pty pair, window size, mode, child-side setsid/TIOCSCTTY;
  master is CLOEXEC with IUTF8 set
- Command.zig: fork/execvpe with Ghostty's exact child setup (stdio
  dup2, best-effort chdir, rlimit restore, os/runtime pre-exec hooks,
  cgroup handling) and exit status decoding
- shell_integration.zig for bash/zsh/fish/elvish/nushell plus feature
  flags; scripts copied verbatim to resources/shell-integration/
- terminfo generator for xterm-ghostty, byte-identical to Zig's encode
  output (golden file produced with the real Zig code)

This crate opts into unsafe_code = "deny" (fork/exec need it) with
allow + SAFETY comments on minimal items.

Integration left: config-derived option types are local stand-ins;
cgroup D-Bus scope creation is a closure hook; Exec/Termio/Thread/
stream_handler wait for ghostty-terminal. Pty now closes both fds on
drop (Zig never auto-closed the slave) - to re-check when wiring Exec.

Test Plan:
- cargo test -p ghostty-termio (52 tests, incl. real pty spawn test
  expecting "hi\r\n31 97\r\n" from `echo hi; stty size`)

resources/shell-integration/nushell/vendor/autoload/ghostty.nu

@@ -0,0 +1,110 @@
+# Ghostty shell integration
+export module ghostty {
+  def has_feature [feature: string] {
+    $feature in ($env.GHOSTTY_SHELL_FEATURES | default "" | split row ',')
+  }
+
+  # Wrap `ssh` with Ghostty TERMINFO support
+  export def --wrapped ssh [...args] {
+    mut ssh_env = {}
+    mut ssh_opts = []
+
+    # `ssh-env`: use xterm-256color and propagate COLORTERM/TERM_PROGRAM vars
+    if (has_feature "ssh-env") {
+      $ssh_env.TERM = "xterm-256color"
+      $ssh_opts = [
+        "-o" "SetEnv COLORTERM=truecolor"
+        "-o" "SendEnv TERM_PROGRAM TERM_PROGRAM_VERSION"
+      ]
+    }
+
+    # `ssh-terminfo`: auto-install xterm-ghostty terminfo on remote hosts
+    if (has_feature "ssh-terminfo") {
+      let ghostty = ($env.GHOSTTY_BIN_DIR? | default "") | path join "ghostty"
+
+      let ssh_cfg = ^ssh -G ...$args
+        | lines
+        | parse "{key} {value}"
+        | where key in ["user" "hostname"]
+        | select key value
+        | transpose -rd
+        | default {user: $env.USER hostname: "localhost"}
+      let ssh_id = $"($ssh_cfg.user)@($ssh_cfg.hostname)"
+
+      if (^$ghostty "+ssh-cache" $"--host=($ssh_id)" | complete | $in.exit_code == 0) {
+        $ssh_env.TERM = "xterm-ghostty"
+      } else {
+        $ssh_env.TERM = "xterm-256color"
+
+        let terminfo = try {
+          ^infocmp -0 -x xterm-ghostty
+        } catch {
+          print -e "infocmp failed, using xterm-256color"
+        }
+
+        if ($terminfo | is-not-empty) {
+          print $"Setting up xterm-ghostty terminfo on ($ssh_cfg.hostname)..."
+
+          let ctrl_path = (
+            mktemp -td $"ghostty-ssh-($ssh_cfg.user).XXXXXX"
+            | path join "socket"
+          )
+
+          let remote_args = $ssh_opts ++ [
+            "-o" "ControlMaster=yes"
+            "-o" $"ControlPath=($ctrl_path)"
+            "-o" "ControlPersist=60s"
+          ] ++ $args
+
+          $terminfo | ^ssh ...$remote_args '
+            infocmp xterm-ghostty >/dev/null 2>&1 && exit 0
+            command -v tic >/dev/null 2>&1 || exit 1
+            mkdir -p ~/.terminfo 2>/dev/null && tic -x - 2>/dev/null && exit 0
+            exit 1'
+          | complete
+          | if $in.exit_code == 0 {
+            ^$ghostty "+ssh-cache" $"--add=($ssh_id)" e>| print -e
+            $ssh_env.TERM = "xterm-ghostty"
+            $ssh_opts = ($ssh_opts ++ ["-o" $"ControlPath=($ctrl_path)"])
+          } else {
+            print -e "terminfo install failed, using xterm-256color"
+          }
+        }
+      }
+    }
+
+    let ssh_args = $ssh_opts ++ $args
+    with-env $ssh_env {
+      ^ssh ...$ssh_args
+    }
+  }
+
+  # Wrap `sudo` to preserve Ghostty's TERMINFO environment variable
+  export def --wrapped sudo [...args] {
+    mut sudo_args = $args
+
+    if (has_feature "sudo") {
+      # Extract just the sudo options (before the command)
+      let sudo_options = (
+        $args | take until {|arg|
+          not (($arg | str starts-with "-") or ($arg | str contains "="))
+        }
+      )
+
+      # Prepend TERMINFO preservation flag if not using sudoedit
+      if (not ("-e" in $sudo_options or "--edit" in $sudo_options)) {
+        $sudo_args = ($args | prepend "--preserve-env=TERMINFO")
+      }
+    }
+
+    ^sudo ...$sudo_args
+  }
+}
+
+# Clean up XDG_DATA_DIRS by removing GHOSTTY_SHELL_INTEGRATION_XDG_DIR
+if 'GHOSTTY_SHELL_INTEGRATION_XDG_DIR' in $env {
+  if 'XDG_DATA_DIRS' in $env {
+    $env.XDG_DATA_DIRS = ($env.XDG_DATA_DIRS | str replace $"($env.GHOSTTY_SHELL_INTEGRATION_XDG_DIR):" "")
+  }
+  hide-env GHOSTTY_SHELL_INTEGRATION_XDG_DIR
+}