Replace golangci-lint action and megalinter with make lint-go

The megalinter ghcr.io image pull is denied, and the golangci-lint
action version was inconsistent with the Makefile-pinned version.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

.gitea/workflows/checks.yml

@@ -25,14 +25,7 @@ jobs:
         with:
           go-version-file: go.mod
           check-latest: true
-      - uses: golangci/[email protected]
+      - run: make lint-go
-      - uses: megalinter/megalinter/flavors/[email protected]
-        env:
-          DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          VALIDATE_ALL_CODEBASE: false
-          GITHUB_STATUS_REPORTER: ${{ !env.ACT }}
-          GITHUB_COMMENT_REPORTER: ${{ !env.ACT }}
 
   test-linux:
     name: test-linux