feat: add plan_snapshot field to ClaudeJob schema

Nullable String? column on claude_jobs captures the task's
implementation_plan at claim time — immutable baseline for drift detection.

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

prisma/migrations/20260430171227_add_claude_job_plan_snapshot/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "claude_jobs" ADD COLUMN     "plan_snapshot" TEXT;

prisma/schema.prisma

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