feat: builtin agent session resume via deterministic message reconstruction (#426)

- StepRecord adds edgePrompt field (backward compat: defaults to "")
- StepNode CAS schema includes edgePrompt
- writeStepNode persists ctx.edgePrompt
- buildHistory exposes edgePrompt in StepContext
- buildBuiltinMessages reconstructs multi-turn moderator↔agent conversation:
  system = role prompt + output format (stable prefix)
  per prior visit: user (edgePrompt + inter-step summary) + assistant (output)
  current: user (edgePrompt + recent summary)
- Zero extra persistence — pure function of CAS chain
- Stable prefix for LLM prompt cache hits
- 10 builtin tests pass, all other package tests pass

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

@@ -41,7 +41,15 @@ describe("buildClaudeCodePrompt", () => {
 
   test("includes previous steps as history summary", () => {
     const ctx = makeCtx({
-      steps: [{ role: "planner", output: '{"plan":"do X"}', agent: "hermes" }],
+      steps: [
+        {
+          role: "planner",
+          output: '{"plan":"do X"}',
+          agent: "hermes",
+          detail: "detail-1",
+          edgePrompt: "Create a plan.",
+        },
+      ],
     });
     const result = buildClaudeCodePrompt(ctx);
     expect(result).toContain("## Previous Steps");