provisionArmBin symlinks the shell arm's one allow-listed binary into a
per-sitting bin directory, but a sitting's trials share that directory and
re-provision it each trial, so the second trial's symlinkSync threw EEXIST.
This crashed every shell arm (gitea-axi, tea, raw-api) at more than one
trial per sitting; the gitea-mcp arm was unaffected because its shell is
disabled and it symlinks nothing.
Remove any existing link before re-creating it so provisioning is
idempotent. Add a regression test that provisions the same bin directory
twice and asserts it neither throws nor duplicates the link.
