docs(readme): polish GitHub showcase README

Full rewrite of README.md for portfolio and public GitHub showcase.
Adds professional hero, live demo links, safety badge rail, feature
overview, architecture diagram, screenshot placeholders, validation
evidence section, portfolio story table, honest roadmap, and
full disclaimer.

Also adds docs/showcase/github_about_metadata.md with proposed
repository description, website URL, and topic tags for the
GitHub About panel.

Safety invariants unchanged. No runtime, frontend, backend, or
config files modified.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Melly-999