Document that PRs are squash-merged, not merged

Both files still describe the pre-July policy and tell contributors to polish
each commit message because the branch lands on master as-is. Under squash only
the PR title and description survive, so the advice pointed at the wrong thing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

AGENTS.md

@@ -67,8 +67,9 @@ Before pushing, and when reviewing others, don't skim for bugs:
 - **Co-Authored-By is mandatory for AI-assisted commits.** Carry a
   `Co-Authored-By:` trailer naming the assistant. Attribute there, never in a
   PR-body footer.
-- PRs land as a merge commit; every commit on the branch goes onto master, so
-  keep each commit message clean and meaningful.
+- PRs are squash-merged: one commit per PR lands on master, built from the PR
+  title and description, so those are what the history keeps. The branch's
+  intermediate commits are not preserved.
 
 ## PR descriptions
 - Plain concise prose; lead with what changed and why. No What/Why/How template.

CONTRIBUTING.md

@@ -6,8 +6,8 @@ with an AI assistant? The operational checklist is [AGENTS.md](AGENTS.md).
 ## Pull requests
 
 - One change per PR. Small diffs merge fast.
-- PRs land as a merge commit, so the branch's commits go onto master as-is: keep
-  each commit message clean and explain *why*.
+- PRs are squash-merged: one commit per PR goes onto master, built from the PR
+  title and description, so those are the history. Explain *why* there.
 - Be terse in the PR title and description: name the problem, not the fix, don't
   restate the diff, and calibrate length to the change.
 - Add or update tests for engine changes (`tests/`), and keep CI green.