🤖 refactor: add reusable build-vscode-extension action

Create composite action for building VS Code extension to avoid
duplication between build and release workflows.

Changes:
- Created .github/actions/build-vscode-extension/action.yml
- Updated release.yml to use the composite action
- Added build-vscode-extension job to build.yml (CI workflow)
- Extension now builds on every PR and uploads as artifact

This ensures the extension builds are tested in CI and available
as artifacts for every PR.

_Generated with `cmux`_

Author: ammar-agent

.github/actions/build-vscode-extension/action.yml

@@ -0,0 +1,9 @@
+name: "Build VS Code Extension"
+description: "Build the cmux VS Code extension"
+
+runs:
+  using: "composite"
+  steps:
+    - name: Build VS Code extension
+      shell: bash
+      run: make vscode-ext

.github/workflows/build.yml

@@ -89,3 +89,24 @@ jobs:
           path: release/*.AppImage
           retention-days: 30
           if-no-files-found: error
+
+  build-vscode-extension:
+    name: Build VS Code Extension
+    runs-on: ${{ github.repository_owner == 'coder' && 'depot-ubuntu-22.04-16' || 'ubuntu-latest' }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # Required for git describe to find tags
+
+      - uses: ./.github/actions/setup-cmux
+
+      - uses: ./.github/actions/build-vscode-extension
+
+      - name: Upload VS Code extension artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: vscode-extension
+          path: vscode/cmux-*.vsix
+          retention-days: 30
+          if-no-files-found: error

.github/workflows/release.yml

@@ -70,8 +70,7 @@ jobs:
 
       - uses: ./.github/actions/setup-cmux
 
-      - name: Build VS Code extension
-        run: make vscode-ext
+      - uses: ./.github/actions/build-vscode-extension
 
       - name: Upload VS Code extension to release
         env: