chore(cli-v2): publish JSON Schema for fern.yml

[devin-ai-integration]

Description

Linear ticket: Closes FER-8474

Generate and publish a JSON Schema for fern.yml (CLI v2 config) so that YAML language servers can provide autocomplete and validation in IDEs.

The schema is generated from the Zod FernYmlSchema using Zod v4's toJSONSchema() and committed as fern-yml.schema.json. Once merged, it will be served at https://schema.buildwithfern.dev/fern-yml.json.

The fern init command in CLI v2 already emits the schema reference comment at the top of generated fern.yml files:

# yaml-language-server: $schema=https://schema.buildwithfern.dev/fern-yml.json

Changes Made

• Added packages/cli/config/scripts/generate-jsonschema.mjs — script that converts the Zod FernYmlSchema to JSON Schema via z.toJSONSchema()
• Added fern-yml.schema.json to the repo root (generated output, ~6k lines)
• Added fern-yml:jsonschema npm script and included it in the aggregate jsonschema script in the root package.json
• Added unreleased changelog entry
• Updated README.md generator (if applicable)

Testing

• Verified schema generation produces valid JSON Schema output
• Verified existing generators-yml.json and fern.json are served from repo root files via schema.buildwithfern.dev — same pattern applies to fern-yml.schema.json
• Lint (pnpm lint:biome) and format (pnpm format:check) pass cleanly

Link to Devin session: https://app.devin.ai/sessions/1e2aa4f2a4dc4a35bee9141dd9a16888

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