chore(index): remove contextInjector chat.message hook call

- Remove createContextInjectorHook from imports
- Remove contextInjector variable declaration
- Remove contextInjector["chat.message"] call
- Keep contextInjectorMessagesTransform for synthetic part injection
- Update test: prepend → synthetic part insertion verification

src/features/context-injector/injector.test.ts

@@ -208,7 +208,7 @@ describe("createContextInjectorMessagesTransformHook", () => {
     ],
   })
 
-  it("prepends context to last user message", async () => {
+  it("inserts synthetic part before text part in last user message", async () => {
     // #given
     const hook = createContextInjectorMessagesTransformHook(collector)
     const sessionID = "ses_transform1"
@@ -228,9 +228,12 @@ describe("createContextInjectorMessagesTransformHook", () => {
     // #when
     await hook["experimental.chat.messages.transform"]!({}, output)
 
-    // #then
+    // #then - synthetic part inserted before original text part
     expect(output.messages.length).toBe(3)
-    expect(output.messages[2].parts[0].text).toBe("Ultrawork context\n\n---\n\nSecond message")
+    expect(output.messages[2].parts.length).toBe(2)
+    expect(output.messages[2].parts[0].text).toBe("Ultrawork context")
+    expect(output.messages[2].parts[0].synthetic).toBe(true)
+    expect(output.messages[2].parts[1].text).toBe("Second message")
   })
 
   it("does nothing when no pending context", async () => {