🤖 Optimize setup-cmux action with node_modules caching (#424)

_Generated with `cmux`_

Add `node_modules` directory caching to the setup-cmux action to
dramatically reduce install times when the lockfile hasn't changed.

## Changes

- Added `node_modules` cache before bun install cache
- Both caches use `bun.lock` hash as key for invalidation
- Renamed bun cache key from `bun-` to `bun-cache-` for clarity

## Expected Impact

**Before:** Every CI run executes `bun install`, taking 5-7 seconds even
with bun's native caching

**After:** When `bun.lock` is unchanged, cache hit allows skipping full
dependency installation, reducing to <1 second

## Why This Works

- `node_modules` cache: Restores the full dependency tree
- Bun install cache: Speeds up fresh installs when lockfile changes
- Together: Fast cache hits + fast misses

This is especially beneficial for integration tests which run frequently
during development.

Author: ammar-agent

.github/actions/setup-cmux/action.yml

@@ -8,13 +8,26 @@ runs:
       with:
         bun-version: latest
 
-    - name: Cache bun dependencies
+    - name: Get Bun version
+      id: bun-version
+      shell: bash
+      run: echo "version=$(bun --version)" >> $GITHUB_OUTPUT
+
+    - name: Cache node_modules
+      uses: actions/cache@v4
+      with:
+        path: node_modules
+        key: ${{ runner.os }}-${{ runner.arch }}-bun-${{ steps.bun-version.outputs.version }}-node-modules-${{ hashFiles('**/bun.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-${{ runner.arch }}-bun-${{ steps.bun-version.outputs.version }}-node-modules-
+
+    - name: Cache bun install cache
       uses: actions/cache@v4
       with:
         path: ~/.bun/install/cache
-        key: ${{ runner.os }}-bun-${{ hashFiles('**/bun.lock') }}
+        key: ${{ runner.os }}-bun-cache-${{ hashFiles('**/bun.lock') }}
         restore-keys: |
-          ${{ runner.os }}-bun-
+          ${{ runner.os }}-bun-cache-
 
     - name: Cache Homebrew (macOS)
       if: runner.os == 'macOS'