feat(os): port Ghostty's Linux OS helpers

Ports ZIG/src/os for Linux into ghostty-os: XDG dirs, home dir, hostname,
env/PATH helpers, passwd lookup (via nix), shell detection, resources
dir discovery, string encodings (printf-q, URL percent), systemd
sd_notify, i18n via gettextrs, xdg-open, desktop/cgroup/kernel info,
temp dirs, URI parsing (port of std.Uri incl. Ghostty's extra options),
and a zbus-based Flatpak HostCommand implementation.

The crate stays free of unsafe. Consequences, all tracked as
TODO(integration) in docs/rewrite/notes/ghostty-os.md:
- locale::ensure_locale is a no-op (setlocale/setenv are unsafe; GTK
  calls setlocale itself); setenv/unsetenv are not exposed.
- passwd's Flatpak branch needs the termio pty and falls back to the
  sandbox entry for now.
- FlatpakHostCommand's D-Bus path is untested (no portal here).
Intentional fix: a leading '@' in NOTIFY_SOCKET is treated as an
abstract socket.

macOS/Windows/WASM modules are out of scope.

Test Plan:
- cargo test -p ghostty-os (88 tests)
- cargo clippy -p ghostty-os --all-targets -- -D warnings

crates/ghostty-os/src/dbus.rs

@@ -0,0 +1,36 @@
+/// Returns true if the program was launched by D-Bus activation.
+///
+/// On Linux GTK, this returns true if the program was launched using D-Bus
+/// activation. It will return false if Ghostty was launched any other way.
+pub fn launched_by_dbus_activation() -> bool {
+    // On Linux, D-Bus activation sets `DBUS_STARTER_ADDRESS` and
+    // `DBUS_STARTER_BUS_TYPE`. If these environment variables are present
+    // (no matter the value) we were launched by D-Bus activation.
+    std::env::var_os("DBUS_STARTER_ADDRESS").is_some()
+        && std::env::var_os("DBUS_STARTER_BUS_TYPE").is_some()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::testutil::with_env_lock;
+
+    #[test]
+    fn dbus_activation_env() {
+        temp_env::with_vars(
+            [
+                ("DBUS_STARTER_ADDRESS", Some("x")),
+                ("DBUS_STARTER_BUS_TYPE", Some("session")),
+            ],
+            || assert!(launched_by_dbus_activation()),
+        );
+        temp_env::with_vars(
+            [
+                ("DBUS_STARTER_ADDRESS", Some("x")),
+                ("DBUS_STARTER_BUS_TYPE", None),
+            ],
+            || assert!(!launched_by_dbus_activation()),
+        );
+        with_env_lock(|| {});
+    }
+}