feat(M13): auto-PR via gh CLI after successful push (auto_pr=true)

New src/git/pr.ts helper wraps 'gh pr create'; returns { url } or { error }.
maybeCreateAutoPr() in update-job-status checks product.auto_pr, builds title
from story.code + task.title, writes pr_url to DB. Non-fatal: gh failure logs
a warning and leaves DONE status intact. Also syncs schema: auto_pr on Product,
pr_url on ClaudeJob.

src/git/pr.ts

@@ -0,0 +1,38 @@
+import { execFile } from 'node:child_process'
+import { promisify } from 'node:util'
+
+const exec = promisify(execFile)
+
+export async function createPullRequest(opts: {
+  worktreePath: string
+  branchName: string
+  title: string
+  body: string
+}): Promise<{ url: string } | { error: string }> {
+  const { worktreePath, branchName, title, body } = opts
+
+  try {
+    const { stdout } = await exec(
+      'gh',
+      ['pr', 'create', '--title', title, '--body', body, '--head', branchName],
+      { cwd: worktreePath },
+    )
+    // 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() ?? ''
+    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 =
+      msg.includes('command not found') ||
+      msg.includes('is not recognized') ||
+      msg.includes('ENOENT')
+    if (isNotFound) {
+      return { error: 'gh CLI not found — install GitHub CLI to enable auto-PR' }
+    }
+    return { error: `gh pr create failed: ${msg.slice(0, 300)}` }
+  }
+}