feat: add pushed_at field to ClaudeJob schema

Nullable DateTime column to record when the agent's feature branch was
pushed to origin. Enables the UI to show a 'pushed' state independently
of DONE status.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/migrations/20260501095454_add_claude_job_pushed_at/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "claude_jobs" ADD COLUMN     "pushed_at" TIMESTAMP(3);

prisma/schema.prisma

@@ -260,6 +260,7 @@ model ClaudeJob {
   claimed_at          DateTime?
   started_at          DateTime?
   finished_at         DateTime?
+  pushed_at           DateTime?
   plan_snapshot       String?
   branch              String?
   summary             String?