chore: align biome.json schema with the installed CLI

[ProductOfAmerica]

Closes #37.

The biome.json $schema pinned 2.4.11, but the installed Biome CLI is 2.4.15, so every pnpm lint run emitted a schema-version-mismatch info diagnostic. This bumps the $schema version segment to match the installed CLI. Nothing else changes.

Evidence

Before ($schema = 2.4.11):

biome.json:2:14 deserialize
  i The configuration schema version does not match the CLI version 2.4.15
Checked 109 files. No fixes applied.
Found 1 info.

After ($schema = 2.4.15):

Checked 109 files. No fixes applied.

Zero info diagnostics, exit 0.

Also verified: pnpm lint:fix modifies nothing, pnpm test passes (443/443), pnpm typecheck:test passes.