Hide OpenClaw heartbeat artifacts from chat

[rogerchappel]

Summary

• classify OpenClaw heartbeat prompts, leaked tool calls, HEARTBEAT.md read output, and HEARTBEAT_OK as internal artifacts
• filter those artifacts from OpenClaw session previews before they enter chat state
• keep the chat transcript render path defensive for already-loaded artifacts

Verification

• pnpm test src/lib/openclaw-heartbeat-artifacts.test.ts
• pnpm typecheck
• git diff --check
• push hook ran pnpm typecheck and pnpm build

Notes

OpenClaw docs describe heartbeat as scheduled main-session turns whose default prompt reads HEARTBEAT.md. This PR keeps those internal heartbeat turns out of CrewCMD's user-facing chat transcript without disabling heartbeats.