refactor(open-core): main.py enterprise-loader comment enumerates specific paid modules — guard covers docs only

[vybe]

Summary

The #847 enterprise-loader comment block in src/backend/main.py (~L955-970) names the specific paid modules the private submodule installs. The enterprise-docs standing rule (trinity-enterprise#45) keeps the paid-feature catalog out of public docs, and .github/workflows/enterprise-docs-guard.yml greps only docs/ + CLAUDE.md — code comments are unguarded, so this survives as the one public enumeration of the catalog.

Flagged twice independently during /review of #1448 (which deliberately did not touch it — mechanism-only PR).

Acceptance Criteria

• Reword the main.py comment to the generic mechanism ("installs the private enterprise routers under /api/enterprise/*") without naming modules
• Decide whether the guard should also grep the seam files (src/backend/main.py, src/backend/services/entitlement_service.py) — cheap path filter addition — or document why code stays out of scope