fix(github): accept more app key formats

[tlgimenes]

This updates the GitHub MCP auth helper to normalize GitHub App private keys before signing JWTs. It now accepts raw PEM values, single-line values with escaped newlines, base64-encoded PEM, and JSON-shaped secrets that contain a nested private key. The change also validates the PEM earlier so startup failures surface as a clear configuration error instead of an OpenSSL NO_START_LINE crash. README and .env example docs now include GITHUB_APP_ID and the supported GITHUB_PRIVATE_KEY formats.

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Summary by cubic

Accepts more GitHub App private key formats and validates them early to prevent startup crashes when signing JWTs. Docs now list GITHUB_APP_ID and explain supported GITHUB_PRIVATE_KEY formats.

• Bug Fixes
  • Normalize GITHUB_PRIVATE_KEY: trims quotes, unescapes \n, reads JSON {privateKey|private_key}, and base64-decodes when needed.
  • Validate the PEM with crypto.createPrivateKey and surface a clear "Invalid GITHUB_PRIVATE_KEY" config error instead of an OpenSSL NO_START_LINE failure.
  • Update README and .env.example with GITHUB_APP_ID and key format guidance.

^{Written for commit 082f080. Summary will update on new commits.}