ci(pr-checks): run the gate on pull requests into edge too

ENGAGEMENT.md §7.1 Q8. `pr-checks.yml` triggered only on PRs into `main`, so a
workstream that lands its phases on `edge` before one cutover PR got no CI at
all until the cutover — all nine M12 phase PRs merged without a single run, and
engagement Phase 8 was about to do the same. A phase should fail on its own PR.

Sonar is untouched: `sonarqube.yml` is a push-on-`main` analysis, not a PR gate,
so no phase PR was ever expected to run it.

Co-Authored-By: Claude <noreply@anthropic.com>

.gitea/workflows/pr-checks.yml

@@ -1,5 +1,5 @@
-# Gate every pull request into `main` on lint + unit tests + a debug build, so a
-# broken build can't reach the deployable branch. Debug builds are auto-signed,
+# Gate every pull request into `main` or `edge` on lint + unit tests + a debug
+# build, so a broken build can't reach the deployable branch. Debug builds are auto-signed,
 # so this gate needs no secrets. The signed *release* APK + Gitea release come
 # later (release.yml, M6). See docs/android/PLAN.md §12.
 #
@@ -18,9 +18,14 @@
 
 name: PR Checks
 
+# `edge` is here because a workstream that lands ten phase PRs onto it before one
+# cutover PR into `main` otherwise gets NO CI at all until the cutover — which is
+# exactly what happened to all nine M12 phase PRs, and would have happened again
+# to engagement Phase 8 (ENGAGEMENT.md §7.1 Q8). A phase should fail on its own
+# PR, not inside the cutover window with a whole workstream's diff to bisect.
 on:
   pull_request:
-    branches: [main]
+    branches: [main, edge]
 
 concurrency:
   group: pr-checks-${{ github.ref }}