refactor: all-agentic architecture — roles as pure data, agent binding at runtime

BREAKING: Major architecture change.

- RoleDefinition = pure data (systemPrompt + schema + dryRunMeta)
- AgentFn = (ctx: ThreadContext) => Promise<string>, reads ctx.currentRole
- WorkflowDefinition decoupled from agents, bound via AgentBinding at runtime
- createWorkflow(def, binding, extract) replaces createRoleModerator
- Meta extraction moved into engine loop
- Delete workflow-util-role package (createRole, decorators, extract all gone)
- Role packages become pure data exports
- Agent packages updated to single-arg AgentFn

小橘 <xiaoju@shazhou.work>

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

@@ -18,6 +18,7 @@ describe("buildAgentPrompt", () => {
       start: startTask("fix the bug"),
       steps: [],
       threadId: "01TEST000000000000000000TR",
+      currentRole: { name: START, systemPrompt: "" },
     };
     const text = buildAgentPrompt("You are an agent.", ctx);
     expect(text).toContain("You are an agent.");
@@ -30,6 +31,7 @@ describe("buildAgentPrompt", () => {
     const ctx: ThreadContext = {
       start: startTask("user task"),
       threadId: "01TEST000000000000000000TR",
+      currentRole: { name: "coder", systemPrompt: "" },
       steps: [
         {
           role: "coder",
@@ -53,6 +55,7 @@ describe("buildAgentPrompt", () => {
     const ctx: ThreadContext = {
       start: startTask("first message full: task content here"),
       threadId: "01TEST000000000000000000TR",
+      currentRole: { name: "coder", systemPrompt: "" },
       steps: [
         {
           role: "planner",
@@ -85,6 +88,7 @@ describe("buildAgentPrompt", () => {
     const ctx: ThreadContext = {
       start: startTask("start"),
       threadId: "01TEST000000000000000000TR",
+      currentRole: { name: "c", systemPrompt: "" },
       steps: [
         {
           role: "a",