Adds docs/guides/VSCODE-COPILOT.md covering the OmniCopilot extension: install
from either store, connection setup, what the picker actually shows and why,
the dashboard-in-a-tab mode, and a troubleshooting table.
Documents two contracts that existed in code but nowhere in the docs:
- The ?prefix= query parameter on GET /v1/models, with the warning that
"canonical" omits providers whose alias already is the canonical id — so
"alias" is the safe direction for a de-duplicated list.
- MODELS_CATALOG_PREFIX_MODE in .env.example and ENVIRONMENT.md, matching how
ARENA_ELO_SYNC_ENABLED and PII_REDACTION_ENABLED are already documented.
The fabricated-docs gate cannot see this flag being read, because
resolveFeatureFlag() indexes process.env by key rather than naming it; added
an allowlist entry explaining that, in the style of the existing entries.
Co-authored-by: Xiangzhe <bakryun0718@proton.me>
