ci: switch to ubuntu-latest now that repo is public

Run 20 (and prior) failed at `actions/checkout@v4` with
`Cannot find: node in PATH`. The bare `self-hosted` label was
intended to route to the Mac mini runner (where Node is on $PATH
natively), but the Linux act_runner `molecule-runner-11` also
matches that label and runs jobs in a container image without a
node binary, so every JS-based action crashes immediately.

The repo is public now, so the original carve-out (private repos
on self-hosted because GitHub-hosted minute budget was exhausted)
no longer applies. ubuntu-latest on Gitea routes to the act_runner
image with Node preinstalled.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -6,12 +6,7 @@ on:
     branches: [main]
 jobs:
   build:
-    # Self-hosted Mac mini — this repo is private and the org's
-    # GitHub-hosted minute budget is exhausted (every ubuntu-latest job
-    # dies in 2s with no step output). Per the 2026-04-22 carve-out:
-    # private repos run on self-hosted; public repos use ubuntu-latest
-    # (still free).
-    runs-on: self-hosted
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4