Competing backend-primacy claims, and the default backend's single-agent limitation isn't where the reader chooses #338

Part of #335

What's wrong

The docs give competing claims about which backend is primary, and the default backend's single-agent limitation is not surfaced where the reader actually chooses a backend.

Evidence (file:line)

• docs/QUICKSTART_SBX.md:3 — "Recommended for federal users" (points at the non-default sbx)
• README.md:449 — "msb by default"
• msb.sh:2597 — the opencode-only fact (default backend supports a single agent) is buried in a code comment
• The KNOWN_FAILURE_MODES agent list is unscoped, so it does not clarify which agents apply to the default backend.

Suggested fix

• Add one authoritative "which backend" section that resolves the primacy conflict.
• Surface the default-backend (msb) agent support at the point where the reader chooses a backend, not in a code comment.

Surfaced via an automated audit (Fable) against a public clone; validated against main 2026-08-19. AI-assisted (OpenCode).