fix(assembler): replace unwrap on root.parent with safe fallback

root.parent().unwrap() at main.rs:45 panicked if AGENT_ROOT pointed at a
filesystem root (e.g. AGENT_ROOT=/). Now falls back to root itself via
.unwrap_or(root.as_path()) so the 'OK  <manifest> → <path>' line just
prints the absolute path in that edge case instead of aborting.

_assembler/src/main.rs

@@ -42,7 +42,7 @@ fn main() -> ExitCode {
             Ok(out_path) => {
                 let name = path.file_name().unwrap_or_default().to_string_lossy();
                 match out_path {
-                    Some(p) => println!("OK  {name} → {}", relative_to(&p, &root.parent().unwrap())),
+                    Some(p) => println!("OK  {name} → {}", relative_to(&p, root.parent().unwrap_or(root.as_path()))),
                     None => println!("OK  {name}"),
                 }
             }