Default agentConfig to {} so permissions default propagates

When agentConfig is absent from YAML, .optional() leaves it undefined,
so project.agentConfig?.permissions evaluates to undefined instead of
"skip". Change to .default({}) so Zod always constructs the object and
applies the inner permissions default.

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

packages/core/src/config.ts

@@ -74,7 +74,7 @@ const ProjectConfigSchema = z.object({
   scm: SCMConfigSchema.optional(),
   symlinks: z.array(z.string()).optional(),
   postCreate: z.array(z.string()).optional(),
-  agentConfig: AgentSpecificConfigSchema.optional(),
+  agentConfig: AgentSpecificConfigSchema.default({}),
   reactions: z.record(ReactionConfigSchema.partial()).optional(),
   agentRules: z.string().optional(),
   agentRulesFile: z.string().optional(),