refactor(core): migrate MessageV2 part leaves + ToolPart to Effect Schema

Migrates all 13 part leaf schemas and the FilePartSource union to Effect Schema with derived .zod compatibility. ToolPart also moves, now referencing ToolState directly via Schema.Struct. The Part discriminated union stays Zod in this slice (migrates next). Also exports Schema-backed {Text,File,Agent,Subtask}PartInput with matching identifiers so prompt.ts PromptInput can reference them via .zod without re-inlining the part shapes. CommandInput keeps its (smaller) FilePart shape inlined since the SDK does not name that one with a $ref.

Author: kitlangton

packages/opencode/src/server/routes/instance/session.ts

@@ -882,7 +882,9 @@ export const SessionRoutes = lazy(() =>
           const msg = await runRequest(
             "SessionRoutes.prompt",
             c,
-            SessionPrompt.Service.use((svc) => svc.prompt({ ...body, sessionID })),
+          SessionPrompt.Service.use((svc) =>
+            svc.prompt({ ...body, sessionID } as unknown as SessionPrompt.PromptInput),
+          ),
           )
           void stream.write(JSON.stringify(msg))
         })
@@ -915,7 +917,7 @@ export const SessionRoutes = lazy(() =>
         void runRequest(
           "SessionRoutes.prompt_async",
           c,
-          SessionPrompt.Service.use((svc) => svc.prompt({ ...body, sessionID })),
+          SessionPrompt.Service.use((svc) => svc.prompt({ ...body, sessionID } as unknown as SessionPrompt.PromptInput)),
         ).catch((err) => {
           log.error("prompt_async failed", { sessionID, error: err })
           void Bus.publish(Session.Event.Error, {