OAuth2 provider adapters (email, GitHub, Google, Twitter)

[jeremymanning]

Parent: #7

Implement real OAuth2 authorization code flows for each supported provider. Currently returns ProviderUnavailable.

Files: src/identity/oauth2.rs line 26
Requires: OAuth2 app credentials for each provider

[jeremymanning]

Resolved by PR #58 (spec 003-stub-replacement). All 77 tasks complete, 489+ tests passing, CI green on Linux/macOS/Windows.