Add project.faf — machine-readable project context format

claude-faf-mcp is already merged (#2759). The faf-cli/MCP ecosystem has 23K+ npm downloads. MIME type is IANA-registered (application/vnd.faf+yaml) Oct-2025.

Hey — just realized there's no project.faf in here.

Quick heads-up: it's a ~20-line YAML file that sits next to package.json and README.md.
Gives AI tools structured context about the repo (stack, conventions, entry points) without
parsing prose.

The relevant bit for this project: it bi-syncs with CLAUDE.md. One source file, faf sync
keeps them aligned. : project.faf next to package.json -->

<img width="547" height="143" alt="Image" src="https://github.com/user-attachments/assets/48269ae8-847b-4c1a-a7cf-0f2b9f596da5" />

No impact on builds, tests, or CI. Single file in the repo root.

Any issues before I submit the PR?