feat(agents): add agent override configuration support

- Add AgentName, AgentOverrideConfig, AgentOverrides types
- Implement createBuiltinAgents with disabled_agents and overrides support
- Support oh-my-opencode.json config for:
  - disabled_agents: disable specific built-in agents
  - agents: override model, temperature, tools, permission per agent
- Tools and permission objects are shallow-merged with base config
- Export types for external consumers
- Update README with agent override documentation

src/agents/types.ts

@@ -0,0 +1,12 @@
+import type { AgentConfig } from "@opencode-ai/sdk"
+
+export type AgentName =
+  | "oracle"
+  | "librarian"
+  | "explore"
+  | "frontend-ui-ux-engineer"
+  | "document-writer"
+
+export type AgentOverrideConfig = Partial<AgentConfig>
+
+export type AgentOverrides = Partial<Record<AgentName, AgentOverrideConfig>>