chore: migrate to pnpm and fix default value resolution for coercion schemas

[CatchMe2]

• Migrates package manager from npm to pnpm 11.1.0: updates packageManager
  in package.json, replaces package-lock.json with pnpm-lock.yaml, and
  updates all three GitHub Actions workflows (ci.yml, publish.yml,
  bump-version.yml) to use pnpm/action-setup@v4 with pinned SHAs
• Fixes default not appearing in generated markdown for coercion schemas
  (e.g. z.stringbool().default(false)): Zod v4 places the default on the
  output JSON schema rather than the input schema for these types
• Extends the fix to all metadata fields by merging input and output schemas
  ({ ...outputSchema, ...inputSchema }), so input takes precedence for
  constraints/enum while output fills in gaps for post-coercion fields
• Makes generateMarkdown async and awaits validate() to support async
  Standard Schema implementations; fixes optional detection to use
  !issues per the spec rather than 'value' in result