feat: publish JSON Schema for flaglint.config.json

[Krishan27]

Summary

flaglint.config.json (and the inline Zod schema in src/config.ts) define a validated config shape, but there is no published JSON Schema that editors can use for autocompletion and inline validation. Without it, contributors writing a config file have no IDE guidance and must read the source.

Providing a schema at schema/flaglint-config.schema.json and linking it from the README gives users instant IDE autocompletion in VS Code, WebStorm, and any other editor that supports $schema.

Acceptance criteria

• schema/flaglint-config.schema.json is present and valid JSON Schema (draft-07 or 2020-12)
• It covers all fields currently defined in the Zod schema in src/config.ts:
  • staleThreshold, minFileCount, include, exclude, wrappers, enforce
• A $schema example is shown in the README under the config section:

  {
    "$schema": "https://raw.githubusercontent.com/flaglint/flaglint/main/schema/flaglint-config.schema.json",
    "staleThreshold": 90
  }

• The schema file is included in the npm package (files in package.json)

Implementation notes

• The schema can be authored by hand from the Zod definition in src/config.ts — no codegen needed
• Keep the schema in sync with the Zod schema; a comment in src/config.ts pointing to the schema file makes the coupling explicit

Files likely touched

• schema/flaglint-config.schema.json — new file
• package.json — add schema/ to files
• README.md — add $schema usage example

No logic changes required — pure docs/config artifact. Great for a first contribution.