Forbid Conventional Commits in commit conventions

The repo guide already prescribed the seven-rules format but did
not rule out the Conventional-Commits alternative, and recent
history has drifted into mixing both styles. Several existing
commits use a "fix(scope): ..." prefix that the project does not
consume for any tooling (no changelog generator, no semantic
release, no commit-lint), so the prefix only adds noise and hurts
log readability.

Add an alwaysApply Cursor rule that explicitly bans Conventional
Commits and restates the seven-rules format with concrete good and
bad examples. Update contrib/claude/commit.md with the same
prohibition so the documentation and the rule agree, and
cross-link both from the existing signing rule so an agent reading
git-commit-signing.mdc lands on the style rule too.

Signed-off-by: Émile Ré <emile@probo.com>

contrib/claude/commit.md

@@ -27,6 +27,20 @@ Not every commit needs a body -- a single line is fine when the change is self-e
 Fix typo in third-party assessment prompt
 ```
 
+## No Conventional Commits
+
+This repository **does not** use Conventional Commits (`type(scope): summary`). The seven-rules style above is the only accepted format. Existing Conventional-Commits-style messages in the history are drift and must not be used as precedent.
+
+```text
+# GOOD
+Disconnect observer in cookie-banner load() error path
+
+# BAD -- Conventional Commits prefix
+fix(cookie-banner): disconnect observer in load() error path
+```
+
+The project does not consume the `type(scope):` prefix for any tooling (no changelog generator, no semantic-release, no commit-lint), so the prefix only adds noise. If a future need for machine-readable commit types arises, raise it in a separate change that updates this document first.
+
 ## Signing and Authorship
 
 All commits **must** be signed (`-s -S`):