In act_runner a label is `<name>:<backend>`; `:host` is the execution-
backend schema (run on host, no container), not part of the label. The
runner registered as `macos:host` therefore has the label `macos`, which
is what runs-on must reference.
Gitea only exposes workflow_dispatch for workflows on the default branch
(main); our CI is on `ci`, so the manual-run button/API isn't available.
Add a push trigger (filtered to the probe file) so we can drive the macOS
runner test from the ci branch. workflow_dispatch kept for post-merge.
Manual-dispatch probe on runs-on macos:host to confirm the runner picks up
jobs and report arch / macOS version / Xcode SDK / toolchains (nix, rustup,
cargo) / git+node, before wiring the macOS release leg. Delete once done.