feat: replace edgePrompt null check with isFirstVisit (Phase 2)

- Add isFirstVisit: boolean to AgentContext
- Compute from steps history: !steps.some(s => s.role === role)
- hermes.ts: use isFirstVisit for first-entry vs re-entry logic
- buildInitialPrompt: always append edgePrompt as Moderator Instruction
- edgePrompt is never blanked — always the real moderator instruction
- New tests for first-visit, re-entry, and fallback scenarios

Refs #405, #407, #404

packages/workflow-agent-claude-code/__tests__/claude-code.test.ts

@@ -7,6 +7,7 @@ function makeCtx(overrides: Partial<AgentContext> = {}): AgentContext {
   return {
     threadId: "01JTEST0000000000000000000" as ThreadId,
     edgePrompt: "Proceed with the assigned role.",
+    isFirstVisit: true,
     workflow: {
       roles: {
         developer: {