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.

prisma/schema.prisma

@@ -127,6 +127,7 @@ model Product {
   description        String?
   repo_url           String?
   definition_of_done String
+  auto_pr            Boolean         @default(false)
   archived           Boolean         @default(false)
   created_at         DateTime        @default(now())
   updated_at         DateTime        @updatedAt
@@ -267,6 +268,7 @@ model ClaudeJob {
   pushed_at           DateTime?
   plan_snapshot       String?
   branch              String?
+  pr_url              String?
   summary             String?
   error               String?
   verify_result       VerifyResult?