feat(backend): queue-time runtime/profile validation; job.runtime authoritative in worker

packages/backend/convex/threads.ts

@@ -245,6 +245,9 @@ export const createUserThread = mutation({
     prompt: v.string(),
     baseBranch: v.optional(v.string()),
     requestedBranchName: v.optional(v.string()),
+    runtime: v.optional(
+      v.union(v.literal('codex'), v.literal('opencode'), v.literal('claude')),
+    ),
     materializeEnvFile: v.optional(v.boolean()),
     envFilePath: v.optional(v.string()),
     aiProviderProfileId: v.optional(v.id('aiProviderProfiles')),
@@ -284,6 +287,7 @@ export const createUserThread = mutation({
         jobType: 'user_change',
         baseBranch: args.baseBranch,
         requestedBranchName: args.requestedBranchName,
+        runtime: args.runtime,
         materializeEnvFile: args.materializeEnvFile,
         envFilePath: args.envFilePath,
         aiProviderProfileId: args.aiProviderProfileId,