feat(ci): Add GitHub Actions workflow for integration tests (gt-htlmp.5)

Runs integration tests on PRs that touch install, rig, config, or routing code.
Uses -tags=integration build tag with 5-minute timeout and Go module caching.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/integration.yml

@@ -0,0 +1,36 @@
+name: Integration Tests
+
+on:
+  pull_request:
+    paths:
+      - 'internal/cmd/install.go'
+      - 'internal/cmd/rig.go'
+      - 'internal/config/**'
+      - 'internal/routing/**'
+      - 'internal/cmd/*_integration_test.go'
+      - '.github/workflows/integration.yml'
+
+jobs:
+  integration:
+    name: Integration Tests
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: '1.24'
+          cache: true
+
+      - name: Configure Git
+        run: |
+          git config --global user.name "CI Bot"
+          git config --global user.email "ci@gastown.test"
+
+      - name: Build
+        run: go build -v ./cmd/gt
+
+      - name: Run integration tests
+        run: go test -v -tags=integration -timeout=4m ./...