GRM-20: fix: enforce GRM-N: conventional on master commits and PR titles

CONTRIBUTING.md

@@ -14,6 +14,7 @@ The `GRM-N` prefix is mandatory — CI extracts it for merge messages and Vikunj
 
 ## Commit Format
 
+### Feature branches
 Use **conventional commits** on feature branches:
 
 ```
@@ -25,17 +26,18 @@ docs: improve README
 
 Allowed types: `feat`, `fix`, `chore`, `docs`, `style`, `refactor`, `perf`, `test`, `ci`, `build`, `revert`, `BREAKING CHANGE`.
 
-**Do NOT** include the `GRM-N:` prefix in commit messages on feature branches — it is added automatically during squash merge.
+**Do NOT** include the `GRM-N:` prefix in commit messages on feature branches.
 
-## PR Workflow
+### Master branch (squash merges)
+Squash commits on `master` must follow:
 
-1. Create a branch `GRM-N` or `GRM-N-brief-description` from `master`
-2. Make changes, committing with conventional format
-3. Push and open a PR against `master`
-4. CI runs lint, unit tests, and molecule tests
-5. When all checks pass, add the `ready-to-merge` label
-6. The auto-merge workflow squash-merges with message `GRM-N: <PR title>`
-7. The post-merge workflow updates the Vikunja task
+```
+GRM-N: <conventional commit message>
+```
+
+Example: `GRM-24: fix: resolve molecule idempotence`.
+
+This format is enforced by the auto-merge workflow, which validates the PR title is a conventional commit before squash-merging and prepending the task ID.
 
 ## Local Testing