fix(context-injector): prefix synthetic part ids with prt_

Ensure injected synthetic message parts satisfy OpenCode's part-id schema and avoid new-session Zod validation failures.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -106,6 +106,8 @@ describe("createContextInjectorMessagesTransformHook", () => {
     expect(
       "synthetic" in secondSyntheticPart && secondSyntheticPart.synthetic === true
     ).toBe(true)
+    expect(typeof secondSyntheticPart.id).toBe("string")
+    expect(secondSyntheticPart.id.startsWith("prt_")).toBe(true)
     expect(secondSyntheticPart.id).toBe(firstSyntheticPart.id)
   })
 

src/features/context-injector/injector.ts

@@ -148,7 +148,7 @@ export function createContextInjectorMessagesTransformHook(
 
       // synthetic part pattern (minimal fields)
       const syntheticPart = {
-        id: `synthetic_hook_${sessionID}`,
+        id: `prt_synthetic_hook_${sessionID}`,
         messageID: lastUserMessage.info.id,
         sessionID: (lastUserMessage.info as { sessionID?: string }).sessionID ?? "",
         type: "text" as const,