feat(step-read): store assembled prompt in CAS, add --prompt flag

Store the fully assembled prompt sent to each agent in CAS as a text
node, referenced from StepNodePayload.assembledPrompt. This enables
exact reproduction of what the agent received for debugging hallucinations.

Changes:
- workflow-protocol: StepRecord + STEP_NODE_SCHEMA add assembledPrompt field
- workflow-util-agent: AgentRunResult includes assembledPrompt, run.ts stores it
- workflow-util-agent: schemas register TEXT_SCHEMA for prompt storage
- workflow-agent-claude-code: return assembled prompt from buildClaudeCodePrompt
- workflow-agent-hermes: return assembled prompt from buildHermesPrompt
- workflow-agent-builtin: return empty prompt (no prompt assembly)
- cli-workflow: step read --prompt renders the stored prompt
- All test fixtures updated for new field

Legacy steps without assembledPrompt show 'Prompt not recorded' message.

小橘 🍊

package.json

@@ -28,7 +28,8 @@
     "@uncaged/workflow-agent-hermes": "workspace:*",
     "bun-types": "^1.3.13",
     "typescript": "^5.8.3",
-    "vitest": "^4.1.7"
+    "vitest": "^4.1.7",
+    "yaml": "^2.9.0"
   },
   "repository": {
     "type": "git",