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/kernel_info.rs

@@ -0,0 +1,29 @@
+use std::io::Read;
+
+/// Returns the kernel release string (`/proc/sys/kernel/osrelease`).
+pub fn get_kernel_info() -> Option<String> {
+    let file = std::fs::File::open("/proc/sys/kernel/osrelease").ok()?;
+
+    // 128 bytes should be enough to hold the kernel information. Zig's
+    // readToEndAlloc fails if the file is larger than the limit.
+    let mut buf = Vec::new();
+    file.take(129).read_to_end(&mut buf).ok()?;
+    if buf.len() > 128 {
+        return None;
+    }
+    let s = String::from_utf8_lossy(&buf);
+    Some(s.trim_matches(|c: char| c.is_ascii_whitespace()).to_owned())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn read_proc_sys_kernel_osrelease() {
+        let kernel_info = get_kernel_info().unwrap();
+        // Since we can't hardcode the info in tests, just check
+        // if something was read from the file
+        assert!(!kernel_info.is_empty());
+    }
+}