docs: add hero demo video playback workflow

[ThomasK33]

Summary

• document the Hero Demo video playback workflow for GitHub README-friendly recordings
• add task-scoped mise tooling for the real-agent Hero Demo recorder/upload asset workflow
• add a helper to prepare H.264 MP4 upload assets and apply GitHub user-attachment URLs back to README/manifest files
• preserve and validate the new playback guide as part of the canonical Hero Demo bundle

Validation

• mise run demo:agent-uses-agent-tty:upload-assets
• npm run format:check
• npm run lint
• npm run typecheck
• npm run validate-bundle -- dogfood/agent-uses-agent-tty --profile canonical
• npx vitest run test/unit/tools/hero-demo.test.ts
• GIT_CONFIG_GLOBAL=/dev/null mise run ci

Note: local mise run ci without GIT_CONFIG_GLOBAL=/dev/null was blocked by the workspace's global Git signing config pointing at a missing 1Password SSH signing binary; rerunning with the global Git config disabled passed.

---

Generated with mux • Model: openai:gpt-5.5 • Thinking: xhigh