fix: auth robustness and docs consistency

[bmdavis419]

Summary

• treat empty auth.json as valid empty state
• reduce noisy auth read warnings
• align runtime + docs auth matrix for providers

Issues

• Closes [Bug?] "Failed to read auth.json" when its empty #184
• Closes [Docs] Incorrect auth information for google #185

Greptile Summary

Improves auth robustness by treating empty auth.json as valid empty state and aligns documentation with runtime behavior for Google provider authentication.

• Added empty file handling in auth file readers to prevent JSON parsing errors
• Removed Google OAuth support from provider auth matrix (API key only)
• Updated all documentation to consistently reflect Google requires API key authentication
• Reduced noisy warnings when reading empty auth files

Confidence Score: 5/5

• Safe to merge with no breaking changes to existing features
• All changes are consistent, well-tested patterns for handling edge cases. The Google OAuth removal only affects documentation since OAuth wasn't actually implemented for Google provider. Empty auth.json handling prevents errors without changing existing behavior.
• No files require special attention

Important Files Changed

Filename	Overview
apps/cli/src/lib/copilot-oauth.ts	Added empty auth.json handling to prevent JSON parsing errors
apps/cli/src/lib/opencode-oauth.ts	Added empty auth.json handling to prevent JSON parsing errors
apps/server/src/providers/auth.ts	Added empty auth.json handling and removed Google OAuth support (API key only)

_{Last reviewed commit: f403623}