When an agent is running, the gateway runner's running-agent block routes
"session-level toggles that are safe to run mid-agent" through a membership
set before the catch-all that rejects everything else with
"Agent is running — /<cmd> can't run mid-turn".
A dedicated /footer dispatch branch already sat inside that guard, but the
set listed only {"yolo", "verbose"}, so the footer branch was unreachable:
/footer fell through to the catch-all and was rejected, forcing users to
/stop a running agent just to toggle the runtime-metadata footer. /footer
is a pure display toggle like its sibling /verbose — it only writes
display.runtime_footer.enabled and returns a status string — so it belongs
in the same set.
Add "footer" to the set so the existing dispatch branch becomes reachable.
Regression test (tests/gateway/test_footer_command_mid_run.py): asserts
/footer and /footer <arg> dispatch to _handle_footer_command while an agent
is running, with a /verbose parity guard. Verified failing before the fix
(handler awaited 0 times) and passing after.
