[Improve] Add free-text environment and model routing guidance - #2485

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Replaced the structured Routing Rules editor with one free-text textarea for environment, model, and reasoning guidance.
• Added a one-time database migration that converts legacy rules in order, resolves readable environment names, preserves unavailable environment IDs, and skips settings that already contain text guidance.
• Kept legacy rule data alongside migrated or newly saved guidance for N-1 rollback safety, while current UI and Fast reads use only the text field.
• Feed free-text guidance into the central Fast prompt while preserving explicit user-choice precedence, enabled target validation, and permission boundaries.
• Added regression coverage for migration reruns, existing-text preservation, save/load/clear behavior, model-only guidance, and Fast prompt wiring.
• Updated the public environment documentation for the new guidance format.

Why this change was made

Structured rules required an environment target, which prevented administrators from expressing model-only routing preferences. A single guidance field supports both routing dimensions, and migrating existing settings during deployment avoids requiring an administrator to save them again.

Impact

Administrators can describe environment and model routing in one place. During upgrade, the database migration converts every legacy structured setting before application services start; existing text is untouched and rerunning the migration is a no-op.