feat(auth): include linked GitHub account id in OAuth id_token

[mroderick]

Summary

Related: codebar/planner#2805.

The planner now matches returning members by the stable GitHub account id when a user's GitHub email differs from their stored planner email (codebar/planner#2806). This change adds that github_id claim to the OAuth id_token so the planner can perform the lookup.

Changes

• Added src/auth/id-token-claims.js to look up a user's linked GitHub accountId from the Better Auth account table.
• Configured customIdTokenClaims on the OAuth provider to inject github_id into the id_token.
• Added integration tests verifying the claim is present for GitHub-linked users and absent for magic-link-only users.

Deployment note

This should be deployed before or alongside codebar/planner#2806. When the claim is absent, the planner falls back to its existing email-based matching.

Post-Deploy Monitoring & Validation

• Monitor logs for errors from getGithubAccountId after deploy.
• Verify that planner logins via codebar auth still work.
• Check that the id_token issued for GitHub-linked users contains a github_id claim.

[olleolleolle]

I think it looks lovely.

[till]

I would do min what @olleolleolle said.

Otherwise I wonder if the custom email lookup broke it for many? You got an approval though, maybe still worth a try.