feat: Check in schema & add sync script

[bsmth]

Description:

Check in the upstream schema and add a script to write it to the repo. Additional automations incoming (cron, PR on diffs). Details in #177

Motivation:

The docs will benefit from a local source of truth for policy definitions. Checking it in makes the schema reviewable & easier to author checks.

Additions:

• schema/policies-schema.json: The upstream schema file
• schema/.source.json: A meta file showing URL & SHA of the schema file
• scripts/sync-schema.mjs: A small utility to fetch & write the schema to this repo
• .prettierignore excludes schema/: stop the formatter from changing the file

Changes:

• package.json: wire in the fetch script via schema:sync

Testing:

To test, run:

npm run schema:sync

Expected result:

tree -a schema
# schema
# ├── .source.json
# └── policies-schema.json

Related issues and pull requests:

Part of #177