ci: trigger macos-probe on push (workflow_dispatch needs default branch)

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.

.gitea/workflows/macos-probe.yaml

@@ -1,13 +1,21 @@
 # THROWAWAY DIAGNOSTIC for the macOS (Tart) runner — delete once the macOS
-# release leg is wired. Manual dispatch only: the Mac isn't always on, so this
+# release leg is wired.
-# runs when you bring it up and trigger it from the Actions UI.
+#
+# Trigger: runs on a push that touches THIS file. workflow_dispatch is kept too,
+# but Gitea only exposes manual dispatch for workflows on the DEFAULT branch
+# (main) — and all our CI is still on `ci` — so push is how we drive it for now.
+# Bring the Mac up before pushing a change here so the run isn't left queued.
 #
 # Answers: does the `macos:host` runner pick up jobs, does it run on the host,
 # what arch + macOS version, is the Xcode SDK present (needed to link arboard's
 # AppKit), and which build toolchain is available (nix? rustup? bare cargo?) —
 # plus git/node for actions/checkout.
 name: macos-probe
-on: [workflow_dispatch]
+on:
+  push:
+    paths:
+      - '.gitea/workflows/macos-probe.yaml'
+  workflow_dispatch:
 
 jobs:
   probe: