Whole-repo routing

Make every command run under the sandbox by default — a transparent shim or a single switch that prefixes virrun -- onto all of install / build / test / dev repo-wide, instead of adopting one command at a time.

Why deferred: Three shipped-state facts make this wrong today — not a design that can be flipped on:

• Auto still resolves to native. No isolating backend has beaten the gates by default, so a blanket prefix runs on the host with zero isolation and zero speedup — pure process-spawn overhead.
• The os backend is ephemeral and isolated by design. Writes land in an invisible tmpfs upper discarded on dispose (write-back only reconciles a mutation command's produced files, not a dev server's live state). Side-effecting long-running commands cannot be transparently wrapped — that is the whole point of isolation, and it is incompatible with "wrap everything."
• The adoption principle forbids it. "Never migrate the repo. Migrate one command." Whole-repo routing throws away per-command reversibility and the benchmark/correctness datapoint each rung produces (adoption).

Revisit when: A viable transparent-interception seam exists. The PATH shim is measured unviable for pnpm-local tools (pnpm prepends .bin ahead of anything inherited), so the natural next rung is a spawn-level interceptor (NODE_OPTIONS, or shimming pnpm itself) — and that, unlike the prefix, would need a committed allowlist to decide which no-prefix commands sandbox. Until such a seam proves out, per-command prefixing stays the mechanism.

Cheaper interim: The adoption ladder already covers the need — prefix one command, promote to a package.json script, commit the config backend selection — scoped to commands proven on the gates.