Fix Windows compatibility in eval framework: file URL scheme and stdin prompt delivery

[Copilot]

Windows users reported two critical failures when running eval experiments: ESM loader rejecting absolute paths (ERR_UNSUPPORTED_ESM_URL_SCHEME: Received protocol 'c:') and Claude Code CLI only receiving "1" instead of the full prompt.

Changes

File URL conversion for dynamic imports

• Added pathToFileURL to convert absolute paths to file:// URLs before dynamic import()
• Affected 6 files with JSON imports: eval.ts, collect-args.ts, prepare-experiment.ts, prepare-evaluations.ts, test-stories.ts, chromatic.ts

// Before
await import(path.join(evalPath, 'hooks.ts'))

// After  
import { pathToFileURL } from 'node:url';
await import(pathToFileURL(path.join(evalPath, 'hooks.ts')).href)

Claude Code CLI prompt delivery

• Moved prompt from command args to stdin (claude-code-cli.ts)
• Ensures complete prompt transmission on Windows

// Before
const args = ['--print', '--verbose', prompt];

// After
const args = ['--print', '--verbose'];
claudeProcess.stdin.write(prompt);

Cross-platform compatible: pathToFileURL handles OS-specific file URL formatting automatically.

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[JReinhold]

I'm not a huge fan of this change, it feels more brittle to input the prompt via stdin afterwards instead of via this flag, which is the documented way of doing it.

we should try to find a different approach that still works on Windows.