ci: gate PR titles via the required linter job (#671)

## Problem

The standalone `pr-title.yml` workflow produced a `Conventional PR
title` status check, but that context was **not** in `main`'s
branch-protection required list (`tests`, `linter`, `static-analysis`,
`browser-tests`, `performance-tests`). Auto-merge only waits on required
checks, so a **failing** title check never blocked a merge — which is
exactly what happened.

## Fix

Fold the same validation into the `linter` job, which is already a
required check. A bad PR title now fails `linter` → blocks the merge
(and auto-merge waits). The standalone workflow is removed as redundant.

Since required checks live in branch protection (not in the repo — no
config-as-code here), hanging the validation off an already-required job
is the only way to enforce it via a PR.

## Trade-off

The title re-validates on `push`/`synchronize` rather than on the
`edited` event, so correcting a bad title after opening a PR needs a new
push (a re-run won't pick up the edited title). Acceptable for the
simpler, single-mechanism setup.

.github/workflows/ci.yml

@@ -235,6 +235,9 @@ jobs:
 
   linter:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: read
     steps:
       - uses: actions/checkout@v4
 
@@ -273,6 +276,33 @@ jobs:
       - name: Frontend Tests
         run: bun run test
 
+      # Gates merges on a Conventional-Commit PR title. Lives in the linter
+      # job (a required check) so a bad title blocks the merge, unlike the old
+      # standalone workflow whose check wasn't required. Skipped on push since
+      # there's no PR title to validate then.
+      - name: Conventional PR title
+        if: github.event_name == 'pull_request'
+        uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          types: |
+            feat
+            fix
+            chore
+            docs
+            style
+            refactor
+            perf
+            test
+            build
+            ci
+            revert
+          requireScope: false
+          subjectPattern: ^(?![A-Z]).+$
+          subjectPatternError: |
+            Subject must not start with uppercase. Use: feat: add checkout, fix(billing): handle retry.
+
   performance-tests:
     runs-on: ubuntu-latest
     services:

.github/workflows/pr-title.yml

@@ -1,34 +0,0 @@
-name: PR title
-
-on:
-  pull_request:
-    types: [opened, edited, synchronize, reopened, ready_for_review]
-
-jobs:
-  semantic-pr-title:
-    name: Conventional PR title
-    runs-on: ubuntu-latest
-    permissions:
-      pull-requests: read
-    steps:
-      - name: Validate PR title
-        uses: amannn/action-semantic-pull-request@v5
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        with:
-          types: |
-            feat
-            fix
-            chore
-            docs
-            style
-            refactor
-            perf
-            test
-            build
-            ci
-            revert
-          requireScope: false
-          subjectPattern: ^(?![A-Z]).+$
-          subjectPatternError: |
-            Subject must not start with uppercase. Use: feat: add checkout, fix(billing): handle retry.