[misc] Load AGENTS.md every session and refuse attribution trailers (#7544)

AGENTS.md forbids attribution trailers, but a rule an agent has to go and read loses to the instruction it is handed every turn. CLAUDE.md now imports AGENTS.md so it is always in context; a commit-msg hook (via make setup-hooks) refuses the trailers at commit time; a CodeRabbit pre-merge check flags a PR whose description or commits carry them. The check reports rather than blocks, since the repository keeps CodeRabbit's request-changes workflow off; turning that on is a separate, repository-wide decision.

.coderabbit.yaml

@@ -14,5 +14,15 @@ reviews:
     - "!**/*.ts"
     - "!**/*.js"
     - "!**/*.svg"
+  pre_merge_checks:
+    custom_checks:
+      - name: "No attribution trailers"
+        mode: error
+        instructions: >-
+          Fail when the PR description or any commit message carries an
+          attribution trailer or footer: Co-Authored-By, Claude-Session,
+          Generated-By, or a "Generated with"/"Generated by" tool line.
+          Contributors own their contributions (AGENTS.md); ask for the
+          lines to be removed.
 chat:
   auto_reply: true