fix(slack): load startup instrumentation in production

[shrimalmadhur]

Summary

• add a root instrumentation entry so Next includes the startup hook in the production server bundle
• reuse the existing server startup logic in src/instrumentation.ts so Slack and issue transports start when the Bun-served app boots

Changes

• add root-level instrumentation.ts that re-exports register from src/instrumentation

Test Plan

• ./node_modules/.bin/tsc --noEmit
• rebuild and restart the installed service so the standalone bundle includes the root instrumentation entry
• verify server logs show the Slack socket manager starting on boot