[Docs] Illustrate cloud/security/keychains with Zara's Infrastructure team keychain

## Overview
Update `cloud/security/keychains` to use Zara Osei configuring a keychain scoped to the Orbital Labs Infrastructure team, controlling access to the `prod-aws` environment.

## Tasks
- [ ] Add scenario: Zara creates a keychain for the Infrastructure team
- [ ] Show keychain scoped to `orbital-production` workspace and `prod-aws` environment
- [ ] Add screenshot (log in as Zara: `zara@orbital-labs.example`) per screenshot standards at `/cloud/about/five-narrative-plan`
- [ ] Link to `/cloud/about` for cast context

## Reference
- Narrative: `/cloud/about`
- Difficulty: Medium