fix(ci): restore the stable "Dogfood Regression Gate" required-check name (#3622 follow-up) (#3644)

INCIDENT: every open PR in the repo is BLOCKED — mergeable, all checks
green, merge button dead.

Root cause: #3622 sharded the dogfood job 2-way, so its checks now
publish as "Dogfood Regression Gate (1/2)" / "(2/2)". Branch protection
still requires the bare context "Dogfood Regression Gate", which no job
emits anymore — the requirement can never be satisfied. #3622's own
comment called for updating branch protection; that step never happened.

Fix keeps the contract in CODE rather than in protection settings: a
tiny aggregator job carrying the stable name, gated on the matrix's
aggregate result. A future shard-count change therefore cannot deadlock
the repo again. `if: always()` + result inspection so a legitimately
skipped matrix (filter says no core paths changed) still satisfies the
gate.

Verified: workflow YAML parses; job graph has dogfood-gate needs:
dogfood with name "Dogfood Regression Gate".

Co-authored-by: Jack Zhuang <277994282+os-zhuang@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: os-zhuang

.github/workflows/ci.yml

@@ -247,6 +247,35 @@ jobs:
             echo "::endgroup::"
           done
 
+  dogfood-gate:
+    # Stable required-check name for a SHARDED job (#3622 follow-up).
+    #
+    # Branch protection requires the context "Dogfood Regression Gate". Once
+    # the job became a 2-way matrix its checks publish as "Dogfood Regression
+    # Gate (1/2)" / "(2/2)" — the bare context could never appear again, so
+    # EVERY pull request in the repo sat permanently BLOCKED (mergeable, all
+    # checks green, merge button dead). #3622's own comment called for updating
+    # branch protection; keeping the contract HERE instead means a future
+    # shard-count change cannot deadlock the repo a second time.
+    #
+    # `if: always()` + result inspection so a legitimately skipped matrix (the
+    # `filter` job says no core paths changed) still satisfies the gate.
+    name: Dogfood Regression Gate
+    needs: dogfood
+    if: always()
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Verify dogfood shard results
+        run: |
+          result="${{ needs.dogfood.result }}"
+          echo "dogfood matrix aggregate result: $result"
+          case "$result" in
+            success|skipped) echo "Dogfood gate satisfied." ;;
+            *) echo "::error::Dogfood shards did not pass (aggregate result: $result)"; exit 1 ;;
+          esac
+
   build-core:
     name: Build Core
     needs: filter