The callback records a typed pending link (short TTL, single use) and
writes no linked identity; POST /v1/apps/:appSlug/links/confirm with the
Heimdall access token of the initiating account attaches it. A victim who
completes an attacker's link URL attaches nothing. The sign-in callback
also evaluates the account's other linked identities with the start-time
policy, so a Discord-origin member returning via Google keeps app_access.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>