feat: use email-based profile IDs for OAuth providers

Changes writeOAuthCredentials and applyAuthProfileConfig calls to use
the email from OAuth response as part of the profile ID instead of
hardcoded ":default".

This enables multiple accounts per provider - each login creates a
separate profile (e.g., google-antigravity:user@gmail.com) instead
of overwriting the same :default profile.

Affected files:
- src/commands/onboard-auth.ts (generic writeOAuthCredentials)
- src/commands/configure.ts (Antigravity flow)
- src/wizard/onboarding.ts (Antigravity flow)

src/wizard/onboarding.ts

@@ -462,7 +462,7 @@ export async function runOnboardingWizard(
       if (oauthCreds) {
         await writeOAuthCredentials("google-antigravity", oauthCreds);
         nextConfig = applyAuthProfileConfig(nextConfig, {
-          profileId: "google-antigravity:default",
+          profileId: `google-antigravity:${oauthCreds.email ?? "default"}`,
           provider: "google-antigravity",
           mode: "oauth",
         });