Update @opencode-ai/plugin and SDK to v1.2.x and align system transform handler signature

- Bump @opencode-ai/plugin ^1.1.19 → ^1.2.16, @opencode-ai/sdk ^1.1.19 → ^1.2.17
- Update system-transform handler input type to match new plugin contract (optional sessionID, required model)
- Add @opencode-ai/sdk override in bun.lock

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/plugin/system-transform.ts

@@ -1,5 +1,5 @@
 export function createSystemTransformHandler(): (
-  input: { sessionID: string },
+  input: { sessionID?: string; model: { id: string; providerID: string; [key: string]: unknown } },
   output: { system: string[] },
 ) => Promise<void> {
   return async (): Promise<void> => {}