ci: post install-test numbers on the pull request

A second workflow, triggered when the install test completes, posts one
comment on the pull request and updates it on every run: install, first
boot and everyday boot next to quattro's latest numbers, with the change,
and the install phases in a collapsed table. Pushes to quattro now run
the install test too, so there is always a baseline.

It runs from quattro's copy with write access, which works for pull
requests from forks, so it never checks out or runs pull request code.
It reads only the finished run's artifact, trusts the pull request
number in it only when that pull request's head is the commit the run
tested, and keeps phase names to plain text.

.github/workflows/install-test.yml

@@ -7,6 +7,9 @@ name: Install test
 
 on:
   pull_request:
+  # Each merge records quattro's numbers, which PR comments compare against.
+  push:
+    branches: [quattro]
   workflow_dispatch:
 
 permissions:
@@ -88,12 +91,19 @@ jobs:
             fi
           } >> "$GITHUB_STEP_SUMMARY"
 
+      # The comment workflow runs from quattro and reads only this artifact;
+      # it needs the PR number, which a workflow_run event omits for forks.
+      - name: Record the pull request
+        if: always() && github.event_name == 'pull_request'
+        run: mkdir -p test-runs && echo "${{ github.event.pull_request.number }}" > test-runs/pr-number
+
       - name: Upload test artifacts
         if: always()
         uses: actions/upload-artifact@b7c566a772e6b6bfb58ed0dc250532a479d7789f # v6
         with:
           name: install-test-${{ github.run_id }}-${{ github.run_attempt }}
           path: |
+            test-runs/pr-number
             test-runs/*/omarchy-install-timing.json
             test-runs/*/omarchy-install.log
             test-runs/*/first-boot-systemd-analyze.txt