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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