core: remove legacy session dependency from v2 API handlers

Eliminates the dependency on LegacySession from the v2 HTTP API,
simplifying the handler initialization by only using SessionV2.
Also exports defaultLayer from SessionV2 for consistent layer provisioning.

Author: thdxr

packages/opencode/src/server/routes/instance/httpapi/v2.ts

@@ -1,4 +1,3 @@
-import { Session as LegacySession } from "@/session/session"
 import { SessionID } from "@/session/schema"
 import { SessionMessage } from "@/v2/session-message"
 import { SessionV2 } from "@/v2/session"
@@ -37,18 +36,16 @@ export const V2Api = HttpApi.make("v2")
     }),
   )
 
-export const v2Handlers = Layer.unwrap(
+export const v2Handlers = HttpApiBuilder.group(V2Api, "v2", (handlers) =>
   Effect.gen(function* () {
-    const legacySession = yield* LegacySession.Service
     const session = yield* SessionV2.Service
-
-    const messages = Effect.fn("V2HttpApi.messages")(function* (ctx: { params: { sessionID: SessionID } }) {
-      yield* legacySession.get(ctx.params.sessionID)
-      return yield* session.messages(ctx.params.sessionID)
-    })
-
-    return HttpApiBuilder.group(V2Api, "v2", (handlers) => handlers.handle("messages", messages))
+    return handlers.handle(
+      "messages",
+      Effect.fn(function* (ctx) {
+        return yield* session.messages(ctx.params.sessionID)
+      }),
+    )
   }),
-).pipe(Layer.provide(LegacySession.defaultLayer), Layer.provide(SessionV2.layer))
+).pipe(Layer.provide(SessionV2.defaultLayer))
 
 export * as V2HttpApi from "./v2"

packages/opencode/src/v2/session.ts

@@ -34,4 +34,6 @@ export const layer: Layer.Layer<Service> = Layer.effect(
   }),
 )
 
+export const defaultLayer = layer
+
 export * as SessionV2 from "./session"