feat(config): add Zod schema validation and JSON Schema generation

- Add Zod schema for oh-my-opencode.json configuration validation
- Generate JSON Schema at build time for IDE autocompletion
- Add safeParse validation with error reporting on config load
- Export OhMyOpenCodeConfigSchema for programmatic usage
- Add build:schema script and ./schema.json export
- Update README with $schema usage documentation

src/config/index.ts

@@ -0,0 +1,15 @@
+export {
+  OhMyOpenCodeConfigSchema,
+  AgentOverrideConfigSchema,
+  AgentOverridesSchema,
+  McpNameSchema,
+  AgentNameSchema,
+} from "./schema"
+
+export type {
+  OhMyOpenCodeConfig,
+  AgentOverrideConfig,
+  AgentOverrides,
+  McpName,
+  AgentName,
+} from "./schema"