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-committer/src/committer.ts

@@ -1,4 +1,4 @@
-import type { AgentFn, Role, ThreadContext } from "@uncaged/workflow";
+import type { AgentFn, Role } from "@uncaged/workflow";
 import {
   createRole,
   decorateRole,
@@ -46,13 +46,9 @@ function resolveExtractDryRun(extractDryRun: boolean | null): boolean {
   return extractDryRun === true;
 }
 
-function committerSystemPrompt(ctx: ThreadContext, config: CommitterConfig): string {
+function committerSystemPrompt(config: CommitterConfig): string {
   return `You are the git committer for this workflow. The project is at \`${config.cwd}\`.
 
-## Context
-
-Use \`uncaged-workflow thread ${ctx.threadId}\` to read the full workflow thread for context on what was done and why.
-
 ## Task
 
 Create a branch, commit the changes, and push. Report whether the push succeeded or failed, the branch name, and the commit SHA.
@@ -77,7 +73,7 @@ export function createCommitterRole(
   const inner: Role<CommitterMeta> = createRole({
     name: "committer",
     schema: committerMetaSchema,
-    systemPrompt: async (ctx) => committerSystemPrompt(ctx, config),
+    systemPrompt: committerSystemPrompt(config),
     agent: adapter,
     extract,
   });