retire the GitHub Actions workflows

Actions are disabled on this repository, so these files have not run in a long
time and cannot run: they are build definitions no forge executes. Builds go
through the native path -- POST /v1/runner, which schedules a BuildKit job --
and CI lives under .hanzo/workflows. A dead workflow that still looks like a
pipeline is worse than none, because it reads as coverage nobody has.

.github/workflows/build-test.yml

@@ -1,39 +0,0 @@
-name: Build and test
-
-on: [push, pull_request, workflow_dispatch]
-
-jobs:
-  build-test:
-    runs-on: ubuntu-latest
-    strategy:
-      max-parallel: 1 # cannot run tests concurrently, files are created
-      matrix:
-        go-version: ['stable', 'oldstable']
-    steps:
-      - uses: actions/checkout@v7
-
-      # Trigger rebuilding frontends, should be the same as committed.
-      - uses: actions/setup-node@v7
-        with:
-          node-version: 24
-          cache: 'npm'
-      - run: 'touch */*.ts'
-
-      - uses: actions/setup-go@v7
-        with:
-          go-version: ${{ matrix.go-version }}
-      - run: make build
-
-      # Need to run tests with a temp dir on same file system for os.Rename to succeed.
-      - run: 'mkdir -p tmp && TMPDIR=$PWD/tmp make test'
-
-      - uses: actions/upload-artifact@v7
-        with:
-          name: coverage-${{ matrix.go-version }}
-          path: cover.html
-
-      # Format code, we check below if nothing changed.
-      - run: 'make fmt'
-
-      # Enforce the steps above didn't make any changes.
-      - run: git diff --exit-code