docs: add commit message length rule (#1373)

FromSi created

## What?

Adds a contribution guideline that commit messages should be 40
characters or fewer.

## Why?

Keeps commit history concise and easier to scan across logs, changelogs,
and review workflows.

Change summary

CONTRIBUTING.md | 2 ++
1 file changed, 2 insertions(+)

Detailed changes

CONTRIBUTING.md 🔗

@@ -60,6 +60,8 @@ type(scope): short description
 
 Common types: `feat`, `fix`, `docs`, `test`, `ci`, `chore`.
 
+Keep commit messages to 40 characters or fewer.
+
 Examples:
 ```
 feat(compose): add CC/BCC field support