chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -1,5 +1,5 @@
 ---
-name: Go
+name: CI
 
 on:
   push:
@@ -8,39 +8,33 @@ on:
     branches: [master]
 
 jobs:
-  # TODO: is it possible to DRY out these jobs? Aside from `runs-on`, they are
-  # identical.
-  # See: https://github.com/actions/runner/issues/1182
-  build-linux:
-    runs-on: [ubuntu-latest]
+  lint:
+    runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - name: Set up Go
-        uses: actions/setup-go@v2
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
         with:
-          go-version: 1.19
-      - name: Set up Revive (linter)
-        run: go get -u github.com/boyter/scc github.com/mgechev/revive
-        env:
-          GO111MODULE: "off"
-      - name: Build
-        run: make build
-      - name: Test
-        run: make test
+          go-version: stable
+      - name: Install revive
+        run: go install github.com/mgechev/revive@latest
+      - name: Lint
+        run: revive -exclude vendor/... ./...
+      - name: Vet
+        run: go vet ./...
+      - name: Check formatting
+        run: test -z "$(gofmt -l . | grep -v vendor/)"
 
-  build-osx:
-    runs-on: [macos-latest]
+  test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v2
-      - name: Set up Go
-        uses: actions/setup-go@v2
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
         with:
-          go-version: 1.19
-      - name: Set up Revive (linter)
-        run: go get -u github.com/boyter/scc github.com/mgechev/revive
-        env:
-          GO111MODULE: "off"
+          go-version: stable
       - name: Build
-        run: make build
+        run: go build -mod vendor ./cmd/cheat
       - name: Test
-        run: make test
+        run: go test ./...