fix(agent-kit): provide full thread context to first-time participating roles

When a role participates for the first time (e.g. committer), it previously
only received the system prompt + last step output, missing the full thread
history. This caused hallucination as the role had to guess what happened.

Changes:
- build-continuation-prompt.ts: detect first-time roles and include all
  steps' meta + content for last 2-3 steps (within quota)
- context.ts: add isFirstVisit detection helper
- types.ts: add isFirstVisit field to AgentContext
- hermes.ts: pass isFirstVisit through to prompt builder

Fixes #473

packages/workflow-protocol/src/types.ts

@@ -63,6 +63,7 @@ export type StepNodePayload = StepRecord & {
 /** JSONata 上下文中的 step — output 被展开 */
 export type StepContext = Omit<StepRecord, "output"> & {
   output: unknown;
+  content: string | null;
 };
 
 export type ModeratorContext = {