Require authorship only in PR descriptions (#11)

Remove commit-message watermark inspection while preserving visible PR attribution, naming, identity exemptions and nonempty commit-range validation.

All79 offline contract cases and shell syntax pass.

Ticket: https://agenthub.fritzlab.net/t-d12vqnp63v5d

action.yaml

@@ -1,6 +1,6 @@
 name: Check PR contract
 description: |
-  Validates that a PR's branch, title, Bug tracking, and AI attribution
+  Validates that a PR's branch, title, Bug tracking, and PR-description AI attribution
   follow the fritzlab Git standard. Fails (exits 1) on any violation, with
   FAIL log lines naming it. Warn-only wiring is the consumer's choice: set
   continue-on-error: true on the job (or pin @v1, which never fails).