security: prevent GitHub username swap attack on 7-day check

[mkrokosz]

Summary

Fixes a vulnerability in the 7-day GitHub account age check that could be bypassed via username swapping.

The Attack:

1. Attacker creates new GitHub account "attacker123" (< 7 days old)
2. Attacker logs into ClaWHub → handle = "attacker123"
3. Attacker tries to publish → fails (account too new)
4. Attacker changes GitHub username to "attacker456"
5. Accomplice (with 7+ day old account) claims "attacker123"
6. Attacker publishes → ClaWHub fetches github.com/users/attacker123 → gets accomplice's old created_at
7. Bypass successful

The Fix:

• Query authAccounts table to get the stored providerAccountId (GitHub's immutable numeric user ID)
• Compare against the id field in the GitHub API response
• Reject if they don't match

Changes

• convex/users.ts: Add getGithubAccountIdInternal query
• convex/lib/githubAccount.ts: Add ID verification check
• convex/lib/githubAccount.test.ts: Add tests for attack scenario

Test plan

• Existing tests pass
• New tests cover attack scenario (ID mismatch rejected, ID match allowed)
• Manual test: publish skill with valid account still works

🤖 Generated with Claude Code

Greptile Overview

Greptile Summary

This PR hardens the 7-day GitHub account age gate against username-swap attacks by looking up the immutable GitHub providerAccountId from authAccounts and comparing it to the id returned by GET /users/:username before trusting created_at. It introduces a new internal query (getGithubAccountIdInternal) and adds unit tests covering both the mismatch (reject) and match (allow) scenarios.

Overall the approach fits cleanly into the existing caching flow in requireGitHubAccountAge: on stale cache, it fetches GitHub metadata, verifies identity, then stores githubCreatedAt/githubFetchedAt on the user.

Confidence Score: 4/5

• This PR is mostly safe to merge and meaningfully improves security, with one fail-open edge case to consider.
• The core fix (binding username to immutable GitHub ID) and tests are straightforward, but the current mismatch logic skips verification when payload.id is missing, which could undermine the intended protection in some scenarios.
• convex/lib/githubAccount.ts

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Context used:

• Context from dashboard - AGENTS.md (source)

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