ci(vex): pass file path positionally to vexctl merge

`vexctl merge` accepts file paths positionally, not via `--files`.
Verified locally with vexctl v0.3.0:

    $ vexctl merge ./out.vex.json
    { ... merged document ... }

Exit 0 with our generated doc confirms parse + schema validation
against the OpenVEX 0.2.0 spec.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mikolalysenko

crates/socket-patch-cli/tests/e2e_vex.rs

@@ -579,7 +579,7 @@ fn maybe_validate_with_vexctl(vex_text: &str) {
     std::fs::write(tmp.path(), vex_text).unwrap();
 
     let out = Command::new(&vexctl)
-        .args(["merge", "--files", tmp.path().to_str().unwrap()])
+        .args(["merge", tmp.path().to_str().unwrap()])
         .output()
         .expect("spawn vexctl");
     assert!(