feat(pr): enable auto-merge (squash) na pr create

Best-effort gh pr merge --auto --squash direct na succesvolle
gh pr create. PR mergt zodra alle vereiste CI-checks groen zijn,
zonder handmatige actie van de gebruiker.

Faal-tolerant: als auto-merge niet werkt (repo heeft "Allow
auto-merge" uit, of token-scope ontbreekt), wordt alleen een
warning gelogd. createPullRequest blijft de PR-URL teruggeven —
auto-merge kan handmatig aangezet worden.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/git/pr.ts

@@ -11,6 +11,7 @@ export async function createPullRequest(opts: {
 }): Promise<{ url: string } | { error: string }> {
   const { worktreePath, branchName, title, body } = opts
 
+  let url: string
   try {
     const { stdout } = await exec(
       'gh',
@@ -19,11 +20,10 @@ export async function createPullRequest(opts: {
     )
     // gh prints the PR URL as the last non-empty line
     const lines = stdout.trim().split('\n').filter(Boolean)
-    const url = lines[lines.length - 1]?.trim() ?? ''
+    url = lines[lines.length - 1]?.trim() ?? ''
     if (!url.startsWith('http')) {
       return { error: `gh pr create produced unexpected output: ${stdout.slice(0, 200)}` }
     }
-    return { url }
   } catch (err: unknown) {
     const msg = (err as { message?: string }).message ?? String(err)
     const isNotFound =
@@ -35,4 +35,21 @@ export async function createPullRequest(opts: {
     }
     return { error: `gh pr create failed: ${msg.slice(0, 300)}` }
   }
+
+  // Best-effort: enable auto-merge (squash) on the freshly created PR. If the
+  // repo doesn't have "Allow auto-merge" turned on, or the token lacks scope,
+  // gh exits non-zero and we just log. The PR is still valid; auto-merge can
+  // be turned on manually. We do NOT fail the whole createPullRequest call —
+  // the URL was successfully obtained which is the contract this returns.
+  try {
+    await exec('gh', ['pr', 'merge', '--auto', '--squash', url], { cwd: worktreePath })
+  } catch (err) {
+    const stderr =
+      (err as { stderr?: string }).stderr ?? (err as Error).message ?? ''
+    console.warn(
+      `[createPullRequest] auto-merge enable failed for ${url}: ${stderr.slice(0, 200)}`,
+    )
+  }
+
+  return { url }
 }