refactor: systemPrompt → pure string, threadId injected by agent layer

- CreateRoleArgs.systemPrompt simplified to string (no more ctx callback)
- buildAgentPrompt injects real ctx.threadId in Tools section
- All four roles compute prompt at construction time from config only
- Removed duplicate ctx.threadId injection from reviewer/committer prompts

小橘 <xiaoju@shazhou.work>

packages/workflow-role-coder/src/coder.ts

@@ -1,4 +1,4 @@
-import type { AgentFn, Role, ThreadContext } from "@uncaged/workflow";
+import type { AgentFn, Role } from "@uncaged/workflow";
 import { createRole } from "@uncaged/workflow-agent-llm";
 import type { LlmProvider } from "@uncaged/workflow-util-role";
 import * as z from "zod/v4";
@@ -40,7 +40,7 @@ export function createCoderRole(
   return createRole({
     name: "coder",
     schema: coderMetaSchema,
-    systemPrompt: async (_ctx: ThreadContext) => coderSystemPrompt(config),
+    systemPrompt: coderSystemPrompt(config),
     agent: adapter,
     extract: {
       provider: extract.provider,