feat: enable GitHub auto-merge for PRs

- Try to enable auto-merge via GitHub API when merge=true
- Fall back to manual merge if auto-merge fails or is not available
- Only perform manual merge if auto-merge was not successfully enabled
- Update log message to clarify when merge is manual vs auto

This allows GitHub to automatically merge PRs when they become ready
(CI passes, approval requirements met), reducing the need for manual
intervention or periodic script re-runs.

Author: ahammond

script.js

@@ -362,6 +362,39 @@ export async function script(
         }
       }
 
+      let autoMergeEnabled = false;
+      if (merge) {
+        try {
+          await octokit.graphql(
+            `mutation enableAutoMerge($pullRequestId: ID!) {
+              enablePullRequestAutoMerge(input: {
+                pullRequestId: $pullRequestId
+                mergeMethod: SQUASH
+              }) {
+                pullRequest {
+                  autoMergeRequest {
+                    enabledAt
+                  }
+                }
+              }
+            }`,
+            {
+              pullRequestId: id,
+            },
+          );
+          octokit.log.info(
+            "auto-merge enabled, GitHub will merge when ready: %s",
+            pr.html_url,
+          );
+          autoMergeEnabled = true;
+        } catch (error) {
+          // Auto-merge not allowed or failed, fall back to manual merge
+          octokit.log.info(
+            `${repository.full_name}: auto-merge not available (${error.message}), falling back to manual merge`,
+          );
+        }
+      }
+
       // Copied from
       // https://github.com/gr2m/octoherd-script-merge-pull-requests/blob/main/script.js
       const result = await octokit.graphql(
@@ -502,7 +535,7 @@ export async function script(
         }
       }
 
-      if (merge) {
+      if (merge && !autoMergeEnabled) {
         const commit_title = `${pr.title} (#${pr.number})`;
         await octokit.request(
           "PUT /repos/{owner}/{repo}/pulls/{pull_number}/merge",
@@ -514,7 +547,7 @@ export async function script(
             merge_method: "squash",
           },
         );
-        octokit.log.info("pull request merged: %s", pr.html_url);
+        octokit.log.info("pull request manually merged: %s", pr.html_url);
       } else {
         octokit.log.info(
           "pull request ready to merge (merge disabled): %s",