feat(context-injector): implement messages transform hook for context injection

- Implement `createContextInjectorMessagesTransformHook` for messages transform hook
- Refactor existing `chat.message` handler to be a no-op (context injection moved to transform)
- Add comprehensive test suite for the new hook (4 test cases)
- Update exports to expose new hook function

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/context-injector/injector.ts

@@ -1,6 +1,6 @@
 import type { ContextCollector } from "./collector"
-
-const MESSAGE_SEPARATOR = "\n\n---\n\n"
+import type { Message, Part } from "@opencode-ai/sdk"
+import { log } from "../../shared"
 
 interface OutputPart {
   type: string
@@ -29,7 +29,7 @@ export function injectPendingContext(
 
   const pending = collector.consume(sessionID)
   const originalText = parts[textPartIndex].text ?? ""
-  parts[textPartIndex].text = `${pending.merged}${MESSAGE_SEPARATOR}${originalText}`
+  parts[textPartIndex].text = `${pending.merged}\n\n---\n\n${originalText}`
 
   return {
     injected: true,
@@ -52,10 +52,115 @@ interface ChatMessageOutput {
 export function createContextInjectorHook(collector: ContextCollector) {
   return {
     "chat.message": async (
-      input: ChatMessageInput,
-      output: ChatMessageOutput
+      _input: ChatMessageInput,
+      _output: ChatMessageOutput
     ): Promise<void> => {
-      injectPendingContext(collector, input.sessionID, output.parts)
+      void collector
+    },
+  }
+}
+
+interface MessageWithParts {
+  info: Message
+  parts: Part[]
+}
+
+type MessagesTransformHook = {
+  "experimental.chat.messages.transform"?: (
+    input: Record<string, never>,
+    output: { messages: MessageWithParts[] }
+  ) => Promise<void>
+}
+
+export function createContextInjectorMessagesTransformHook(
+  collector: ContextCollector
+): MessagesTransformHook {
+  return {
+    "experimental.chat.messages.transform": async (_input, output) => {
+      const { messages } = output
+      if (messages.length === 0) {
+        log("[context-injector] messages.transform: no messages")
+        return
+      }
+
+      const lastMessage = messages[messages.length - 1]
+      const sessionID = (lastMessage.info as unknown as { sessionID?: string }).sessionID
+      if (!sessionID) {
+        log("[context-injector] messages.transform: no sessionID on last message")
+        return
+      }
+
+      const hasPending = collector.hasPending(sessionID)
+      log("[context-injector] messages.transform check", {
+        sessionID,
+        hasPending,
+        messageCount: messages.length,
+      })
+
+      if (!hasPending) return
+
+      let lastUserMessageIndex = -1
+      for (let i = messages.length - 1; i >= 0; i--) {
+        if (messages[i].info.role === "user") {
+          lastUserMessageIndex = i
+          break
+        }
+      }
+
+      if (lastUserMessageIndex === -1) {
+        log("[context-injector] messages.transform: no user message found")
+        return
+      }
+
+      const pending = collector.consume(sessionID)
+      if (!pending.hasContent) {
+        log("[context-injector] messages.transform: pending was empty")
+        return
+      }
+
+      const refMessage = messages[lastUserMessageIndex]
+      const refInfo = refMessage.info as unknown as {
+        sessionID?: string
+        agent?: string
+        model?: { providerID?: string; modelID?: string }
+        path?: { cwd?: string; root?: string }
+      }
+
+      const syntheticMessageId = `synthetic_ctx_${Date.now()}`
+      const syntheticPartId = `synthetic_ctx_part_${Date.now()}`
+      const now = Date.now()
+
+      const syntheticMessage: MessageWithParts = {
+        info: {
+          id: syntheticMessageId,
+          sessionID: sessionID,
+          role: "user",
+          time: { created: now },
+          agent: refInfo.agent ?? "Sisyphus",
+          model: refInfo.model ?? { providerID: "unknown", modelID: "unknown" },
+          path: refInfo.path ?? { cwd: "/", root: "/" },
+        } as unknown as Message,
+        parts: [
+          {
+            id: syntheticPartId,
+            sessionID: sessionID,
+            messageID: syntheticMessageId,
+            type: "text",
+            text: pending.merged,
+            synthetic: true,
+            time: { start: now, end: now },
+          } as Part,
+        ],
+      }
+
+      messages.splice(lastUserMessageIndex, 0, syntheticMessage)
+
+      log("[context-injector] Injected synthetic message", {
+        sessionID,
+        insertIndex: lastUserMessageIndex,
+        contextLength: pending.merged.length,
+        newMessageCount: messages.length,
+      })
     },
   }
 }