Auto-start auth from Cursor session hook

[ishaanxgupta]

Summary

• Start the browser auth flow from Cursor's sessionStart hook when no API key is configured.
• Add auth-attempted and logged-out markers so first-use auth does not nag after a failed/ignored prompt or explicit logout.
• Clear markers on successful/manual login so auth can be retried intentionally.

Testing

• bun run typecheck
• bun run build
• Temporary-home smoke: pre-created .supermemory-cursor/.auth-attempted, ran bun run src/hooks/session-start.ts, confirmed it returned {"continue":true} without launching auth.

Stacked on #2 because the current Cursor auth branch contains the web-app OAuth routing this builds on.