Support safe generic downstream add-ons

[nicko-ai]

Problem

The downstream product add-ons flow must stay generic in agentswarm-cli, while OpenSwarm owns its concrete add-ons list. The first implementation also had unsafe .env edge cases:

• escaped values did not round-trip correctly;
• selected keys could be partially written during prompting;
• duplicate or malformed add-on config could produce wrong prompts;
• new .env files needed secret-file permissions;
• writes needed to refuse tracked .env files;
• existing CRLF line endings needed to be preserved.

Expected

Keep the OpenSwarm-compatible .env add-ons path, but make agentswarm-cli expose only a validated generic add-ons container and safe env writing.

Scope

Replace the hardcoded add-ons list with validated downstream product config, keep /addons hidden by default, harden .env writes, and add focused unit and terminal E2E coverage.