Persist selected frontend agent

[chubes4]

Summary

• Read the Data Machine active-agent preference when loading the frontend chat agent selector.
• Persist selector changes through a small REST proxy to datamachine/set-active-agent.
• Keep the neutral FAB label while the selected agent continues to drive chat routing.

Testing

• npm run typecheck
• npm run build
• php -l inc/rest.php && php -l inc/config.php

Depends on

• Add active agent preference data-machine#2003 or the Data Machine active-agent preference abilities being available.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the REST proxy and frontend selector persistence wiring; Chris reviewed direction and requested the implementation.