build: enable auto merge strategy for pull requests

This commit enables the 'auto' merge strategy for pull requests.
This strategy automatically determines the best merge method based on the
pull request's commits.

The auto merge strategy can:
- Delegate to the autosquash merge strategy if the PR has fixup/squash
  commits against multiple normal commits.
- Squash commits if the PR has only one normal commit and some
  fixup/squash commits.
- Rebase commits if the PR has no fixup/squash commits.

This improves the developer experience by automating the merge process. A
key benefit is that PRs that can be cleanly rebased will now appear as
'merged' in GitHub's UI, providing a clearer history than the previous
'unmerged' status that could occur with squashing.

(cherry picked from commit ffa7266)

Author: alan-agius4

.ng-dev/commit-message.mjs

@@ -9,7 +9,6 @@ export const commitMessage = {
   maxLineLength: Infinity,
   minBodyLength: 0,
   minBodyLengthTypeExcludes: ['docs'],
-  disallowFixup: true,
   // Note: When changing this logic, also change the `contributing.ejs` file.
   scopes: packages.map(({ name }) => name),
 };

.ng-dev/pull-request.mjs

@@ -1,12 +1,12 @@
 /**
  * Configuration for the merge tool in `ng-dev`. This sets up the labels which
  * are respected by the merge script (e.g. the target labels).
- * 
+ *
  * @type { import("@angular/ng-dev").PullRequestConfig }
  */
 export const pullRequest = {
   githubApiMerge: {
-    default: 'rebase',
+    default: 'auto',
     labels: [{ pattern: 'merge: squash commits', method: 'squash' }],
   },
 };