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/cgroup.rs

@@ -0,0 +1,40 @@
+use std::io::Read;
+
+/// Maximum number of bytes read from the cgroup file (the Zig version reads
+/// into a caller-provided buffer; callers there use path-sized buffers).
+const READ_LIMIT: u64 = 4096;
+
+/// Returns the path to the cgroup for the given pid.
+pub fn current(pid: u32) -> Option<String> {
+    // Read our cgroup by opening /proc/<pid>/cgroup and reading the first
+    // line. The first line will look something like this:
+    // 0::/user.slice/user-1000.slice/session-1.scope
+    // The cgroup path is the third field.
+    let file = std::fs::File::open(format!("/proc/{pid}/cgroup")).ok()?;
+    let mut contents = Vec::new();
+    file.take(READ_LIMIT).read_to_end(&mut contents).ok()?;
+
+    // Find the last ':'
+    let idx = contents.iter().rposition(|&b| b == b':')?;
+    let rest = &contents[idx + 1..];
+    let end = rest
+        .iter()
+        .rposition(|b| !matches!(b, b' ' | b'\r' | b'\n'))
+        .map_or(0, |i| i + 1);
+    Some(String::from_utf8_lossy(&rest[..end]).into_owned())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn current_cgroup_of_self() {
+        // Not part of the Zig tests; sanity check that parsing works when
+        // /proc is available.
+        if let Some(p) = current(std::process::id()) {
+            assert!(!p.ends_with('\n'));
+        }
+        assert!(current(u32::MAX).is_none());
+    }
+}