fix: initialize foreground chat gateway before availability checks

[cazador481]

Fixes #2590

Summary

• initialize a cold process-global gateway from the active gbrain config before foreground chat availability checks
• preserve already configured gateways, including engine-resolved model overrides
• cover both gbrain enrich and gbrain extract-conversation-facts foreground paths with a serial regression test

Verification

• bun test test/foreground-chat-gateway-init.serial.test.ts
• bun test test/extract-conversation-facts.test.ts test/enrichment.test.ts
• bun run typecheck
• git diff --check