install task errors on --help via mise run

[ikma-ricon]

Repro from current main (6462aa2):

cd ~/agents/ikma/shiv
mise run install -- --help

Observed:

.mise/tasks/install: line 12: usage_name: unbound variable
[install] ERROR task failed

Expected: help output or at least a clean usage error. This surfaced while checking the tits source registration; not caused by that source-only change.

Likely shape: install task reads usage_name before usage/help handling has populated it for this invocation path, or the task is not declaring args in the canonical #USAGE shape.