feat: add per-cron-job model tier selection

Allow cron jobs to specify a `model_tier` field that controls which LLM
tier handles the job, without needing separate agent configs. Precedence:
cron job model_tier > agent config > global primary_tier > 'default'.

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

src/config/schema.ts

@@ -121,6 +121,8 @@ const mcpSchema = z.object({
   servers: z.array(mcpServerSchema).default([]),
 }).default({ servers: [] });
 
+const modelTierEnum = z.enum(['fast', 'default', 'complex', 'local']);
+
 const cronJobSchema = z.object({
   name: z.string().min(1, 'Cron job name is required'),
   schedule: z.string().min(1, 'Cron schedule is required'),
@@ -131,6 +133,7 @@ const cronJobSchema = z.object({
   }),
   enabled: z.boolean().default(true),
   timezone: z.string().optional(),
+  model_tier: modelTierEnum.optional(),
 });
 
 const webhookSchema = z.object({
@@ -349,8 +352,6 @@ const sandboxSchema = z.object({
 
 // ── Agent config + routing schemas ────────────────────────────────────
 
-const modelTierEnum = z.enum(['fast', 'default', 'complex', 'local']);
-
 const agentConfigEntrySchema = z.object({
   system_prompt: z.string().optional(),
   model_tier: modelTierEnum.optional(),