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-util-agent/__tests__/build-agent-prompt.test.ts

@@ -46,7 +46,7 @@ describe("buildAgentPrompt", () => {
     expect(text).toContain("only step full body");
     expect(text).toContain('Meta: {"files":["a.ts"]}');
     expect(text).toContain("## Tools");
-    expect(text).toContain("uncaged-workflow thread <threadId>");
+    expect(text).toContain("uncaged-workflow thread 01TEST000000000000000000TR");
   });
 
   test("two or more steps: previous steps are meta-only; latest step is full", () => {
@@ -78,6 +78,7 @@ describe("buildAgentPrompt", () => {
     expect(text).toContain("last step full content");
     expect(text).toContain('Meta: {"done":true}');
     expect(text).toContain("## Tools");
+    expect(text).toContain("uncaged-workflow thread 01TEST000000000000000000TR");
   });
 
   test("middle steps show meta summary only, not full content", () => {

packages/workflow-util-agent/src/build-agent-prompt.ts

@@ -41,7 +41,9 @@ export function buildAgentPrompt(systemPrompt: string, ctx: ThreadContext): stri
 
   lines.push("");
   lines.push("## Tools");
-  lines.push("Use `uncaged-workflow thread <threadId>` to read full details of any previous step.");
+  lines.push(
+    `Use \`uncaged-workflow thread ${ctx.threadId}\` to read full details of any previous step.`,
+  );
 
   return lines.join("\n");
 }