fix(issues): add slack ingress diagnostics

[shrimalmadhur]

Summary

• add targeted logging for Slack issue events as they enter the socket handler
• log why Slack mentions or thread replies are ignored, including channel and subtype filters
• log successful Slack issue creation and reply persistence so production triage is visible

Test Plan

• bun test src/lib/issues/tests/telegram-poller.test.ts src/lib/issues/tests/pipeline.test.ts
• send a fresh Slack @mention after deploy and verify the new ingress logs in journalctl