ci: adopt central reusable guards + top-level concurrency

Replace comment/leak/identity guard workflows with thin callers into
freemkv/.github reusable guards (guard-comment/leak/identity @v1); fork
PRs still run, same-repo pushes aren't double-run. Delete now-unused
local ci/comment-guard.py and ci/leak-guard.sh. Add top-level
concurrency (cancel-in-progress) to ci.yml and qa.yml and standardize
actions/checkout to @v5.

.github/workflows/identity-guard.yml

@@ -1,41 +1,14 @@
 name: identity-guard
-# Prevention: no commit may carry an unrelated-business or non-canonical author/
-# committer email. dev@example.com (a dead corp address) once slipped onto the
-# release cascade + a batch of local commits; this fails the build if it recurs.
+
+# Thin caller → the canonical reusable guard in freemkv/.github.
 on:
   push:
     branches: [dev, qa, main]
   pull_request:
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
   identity:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v7
-        with:
-          fetch-depth: 0
-      - name: Reject banned author/committer emails
-        env:
-          BEFORE: ${{ github.event.before }}
-          AFTER: ${{ github.sha }}
-          EVENT: ${{ github.event_name }}
-          PR_BASE: ${{ github.event.pull_request.base.sha }}
-          PR_HEAD: ${{ github.event.pull_request.head.sha }}
-        run: |
-          set -euo pipefail
-          # Unrelated business domains + a personal address normalized to the
-          # GitHub noreply. Real external contributors are unaffected.
-          BANNED='@example-a\.invalid|@example-b\.invalid|owner@example\.invalid'
-          if [ "$EVENT" = "pull_request" ]; then
-            range="$PR_BASE..$PR_HEAD"
-          elif [ -n "${BEFORE:-}" ] && [ "$BEFORE" != "0000000000000000000000000000000000000000" ] && git cat-file -e "$BEFORE" 2>/dev/null; then
-            range="$BEFORE..$AFTER"
-          else
-            range="$AFTER~1..$AFTER"
-          fi
-          bad=$(git log --format='%an <%ae>%n%cn <%ce>' "$range" 2>/dev/null | sort -u | grep -iE "$BANNED" || true)
-          if [ -n "$bad" ]; then
-            echo "::error::commit(s) carry a banned author/committer identity:"; echo "$bad" | sed 's/^/  /'
-            echo "::error::Commit as your GitHub noreply (1085847+MattJackson@users.noreply.github.com). Fix: git config user.email <noreply>; git commit --amend --reset-author."
-            exit 1
-          fi
-          echo "identity-guard: clean"
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.fork
+    uses: freemkv/.github/.github/workflows/guard-identity.yml@v1