fix: require authorship only in PR descriptions

Align the shared naming checker with the Human-directed PR-only attribution policy. Preserve visible PR provenance, naming, tracking, current identity exemptions, available base/head commits and nonempty-range validation without inspecting commit messages.

Verify Human and named-Agent callers against legacy, plain, malformed, misplaced and nonfinal commit trailers; retain missing/malformed PR attribution and invalid-range rejection. All 79 offline contract tests, shell syntax and whitespace checks pass.

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

Policy decision: websites/docs.fritzlab.net#454 (comment)

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).