feat(auth): add token-free git credential helper option

[00200200]

Summary

• add a git_credential_helper action input for token-free Git authentication
• keep the existing credential-helper behavior for allowed_non_write_users
• document the security trade-off and cover the explicit option with a unit test

Fixes #1818

Testing

• npx --yes bun test (967 passed)
• npx tsc --noEmit
• npx prettier --check action.yml docs/security.md docs/usage.md src/github/operations/git-config.ts test/git-config.test.ts
• git diff --check