Document MODELS_ENABLED_BY_DEFAULT availability default

[weselben]

TL;DR

MODELS_ENABLED_BY_DEFAULT exists in config/models.go, .env.template, and
config.example.yaml, but no narrative docs page mentions it. This PR
documents the availability default on the Virtual Models page and in the
advanced configuration reference, including the contrast with the listing-only
KEEP_ONLY_ALIASES_AT_MODELS_ENDPOINT.

---

This PR description was generated with AI assistance.

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

• Autopilot · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

Autopilot is currently an internal CodeRabbit preview.

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}