Add the guard that confines each benchmark arm's agent to exactly one
tool, so a result measures the tool rather than the agent's choice
between tools.
`guardCommand` inspects every binary a proposed shell command would
reach — across pipelines, sequences, subshells, command and process
substitutions, redirections, and leading environment assignments — and
permits only the active arm's one allow-listed binary plus a curated set
of harmless read-only utilities. Foreign binaries, absolute-path
evasions (even of the arm's own binary), and interpreter-based fetch
tricks are denied; the gitea-mcp arm runs with the shell disabled
entirely. `provisionArmBin` produces a curated per-arm bin directory
exposing only that arm's binary as the convenience layer behind the
authoritative guard.
Tests are colocated in bench/guard.test.ts and run via `npm run
test:bench`.
