chore: Update README demo video #430

Summary

Replace or refresh the demo media on the README so it accurately reflects the current Authsome product (hosted UI, principal/vault model, proxy flow, dashboard).

Current state

README.md includes a Demo section with a GitHub-hosted video asset:

## Demo

https://github.com/user-attachments/assets/27f9b229-baf4-4889-be9a-378a133654dc

The rest of the README describes:

• authsome login + authsome run proxy injection flow
• Self-hosting via docker compose
• Agent skill quick start (npx skills add agentrhq/authsome)
• Bundled providers and agent integrations

Why update

The attached demo may predate significant product changes:

• Hosted account UI (email/password, claim flow)
• Dashboard pages: Providers, Connections, Agents, Principals, Audit, Settings
• Principal/Vault/Identity domain model
• Docker/Postgres/Redis self-hosting path
• Device code and browser-bridge auth flows

A stale demo missets expectations for new users and contributors.

Proposed work

• Record a new end-to-end demo (recommended flows):
  • Quick start: skill install → agent task requiring external API
  • OR hosted: docker compose up → register account → claim identity → login provider → authsome run
• Keep video concise (60–90s) with captions or on-screen labels.
• Upload asset to GitHub user-attachments or repo assets/ (gif/mp4).
• Update README Demo section with new asset URL.
• Optionally add a static screenshot fallback for README renderers that block video.

Relevant files

• README.md
• assets/ (if storing locally)
• docs/site/quickstart.mdx — keep aligned with demo narrative

Acceptance criteria

• README demo shows current UI and CLI flows.
• Demo matches documented quick-start paths (local CLI and/or self-hosted).
• No broken media links on github.com render of README.