[Feature] Phase 4c: Third-party connector catalog (Zapier/n8n-style)

[PenguinzTech]

Starter set on the YAML-manifest + node_generator framework: Slack, Discord, MS Teams, Telegram, SMTP email, GitHub, GitLab, Jira, PagerDuty, Opsgenie, Twilio, Google Sheets/Drive, S3-compatible, generic Webhook/HTTP, OpenAI-compatible AI. Connectors double as diagram nodes. Finish half-built connector config/test endpoints; credentials via penguin-sal + OAuth client subsystem.

Spec: docs/design/2026-07-07-elder-platform-merge-design.md (release/v4.0.X)