- Add normalizePrivateKey() to handle literal \n, \r\n, raw base64,
  single-line PEM, PKCS#1 format, and invalid key errors
- Integrate normalization into HttpClient constructor for early validation
- Add checkout integration section to README (3-step flow, new-tab recommendation)
- Add Private Key Formats section to README Configuration
- Bump version to 0.1.3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>