feat: parse_headers.sh and armor.yml update

[ayukum-hyd]

This PR introduces a minimal header parsing pipeline and aligns the CI workflow:

• Replaces the previous parser with a minimal parse_headers.sh that emits only the files required by downstream steps:

  • headers.txt (union of all headers; repo‑relative)
  • blocking_headers_final.txt (repo‑relative)
  • nonblocking_headers_final.txt (repo‑relative)

• Updates armor.yml to:

  • Read config.yml from PR HEAD so changes in the PR are respected.
  • Intersect Git diff changes with the new headers.txt.
  • Copy the *_final.txt lists into $GITHUB_WORKSPACE (if outputs are elsewhere) for run_armor.sh.

This reduces CI noise, removes fragile path handling, and makes the pipeline faster and more reliable.