[Demo][Copilot SDK] Add internal release-notes assistant endpoint

## Demo goal
Anchor the Copilot SDK section with a practical internal platform use case.

## Requested change
Create `demo-app/sdk-prototype/README.md` with:
- Architecture diagram (text is fine)
- Endpoint concept: `/assist/release-notes`
- Data inputs (merged PRs, labels, commit summaries)
- Pseudocode in one SDK language (Node.js preferred)
- Rollout plan and guardrails

## Presenter cue
Use this issue to explain "embed Copilot in internal developer portal" strategy.

Start with demo-app/sdk-prototype/README.md and structure it around the requested architecture, /assist/release-notes endpoint, data inputs, Node.js pseudocode, rollout plan, and guardrails. Done means the README contains all five sections and gives presenters a clear internal developer-portal use case to explain.

Written by the indexing model from the issue text.