feat(cli): add EverOS demo TUI

[cyfyifanchen]

Summary

• Adds a playable everos demo flow: the default command prompts for one memory and one recall question, then renders that story in the full-screen TUI.
• Keeps everos demo --cinematic for the looping README/media showroom and everos demo --plain for static or non-interactive terminals.
• Reworks the README Quick Start so users can try the demo before API keys or server setup, with refreshed externally hosted SVG screenshot and animation.
• Removes heavy architecture/storage/project-structure blocks from the README and points readers to focused docs instead.
• Adds lightweight product context in PRODUCT.md for future terminal-native design work.

Test Plan

• uv run pytest tests/unit/test_entrypoints/test_cli/test_demo_story.py tests/unit/test_entrypoints/test_cli/test_demo_command.py tests/unit/test_entrypoints/test_cli/test_demo_tui.py tests/unit/test_entrypoints/test_cli/test_demo_readme_media.py -q
• make lint
• make test
• make integration
• uv build --sdist --wheel
• git diff --check
• uv run everos demo --help
• uv run everos demo --plain
• uv run everos demo | tail -n 6