Stdin/pipe input support

Unix philosophy is composition, and composition requires stdin. When mytool validate only accepts --file path.json and refuses piped input, users cannot run curl api.example.com/data | mytool validate or chain your tool into generators. Tools that cannot participate in pipelines get replaced by tools that can. Worse, stdin reading without TTY detection causes interactive invocations to hang forever — the user sees a frozen terminal and assumes the tool crashed.

Accept - as a filename meaning stdin, or auto-detect !process.stdin.isTTY and read from the pipe. Implement once in src/cli/lib/input.ts:

async function getInput(filePath?: string) {
  if (filePath && filePath !== '-') return readFileSync(filePath, 'utf8')
  const chunks: Buffer[] = []
  for await (const chunk of process.stdin) chunks.push(chunk)
  return Buffer.concat(chunks).toString('utf8')
}