fix(auth): add oauth-2025-04-20 beta header + debug logging

Ensure anthropic-beta includes oauth-2025-04-20 when using OAuth tokens,
fixing 401 "OAuth authentication is currently not supported" errors.
Add debug-level logging for upstream requests/responses, sniffed headers,
and token refresh operations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/proxy/sniff.go

@@ -122,6 +122,11 @@ func SniffClaudeCode(claudeBinary string) (*SniffedProfile, error) {
 		Int("headers", len(profile.Headers)).
 		Int("body_size", len(profile.Body)).
 		Msg("sniffed claude-code profile")
+
+	for _, h := range profile.Headers {
+		log.Debug().Str("header", h[0]).Str("value", h[1]).Msg("sniffed header")
+	}
+
 	return profile, nil
 }