🤖 Fix npm publish workflow to skip existing versions (#365)

## Problem

The npm publish workflow on `main` has been failing with:
```
npm error 404  '@coder/[email protected]' is not in this registry.
```

This error is misleading. The actual issue is:
- The workflow runs on **every push to main**
- It tries to publish version `0.3.0` every time
- npm doesn't allow republishing the same version
- The 404 error is npm's confusing way of saying "this version already
exists"

## Solution

Use `git describe` to generate **unique pre-release versions** for each
commit to main. This allows every push to be publishable without version
conflicts.

### Version Format

**On main branch (pre-releases):**
```
0.3.0-next.14.g6c0e10d
└─┬─┘ └─┬─┘ └┬┘ └──┬──┘
  │     │    │     └─ short git hash
  │     │    └─────── commits since last tag
  │     └──────────── pre-release identifier
  └────────────────── base version from package.json
```
Published to npm with `next` tag: `npm install @coder/cmux@next`

**On version tags (stable releases):**
```
0.3.0
```
Published to npm with `latest` tag: `npm install @coder/cmux`

## How It Works

1. **Generate unique version**: Calculate commits since last tag + git
hash
2. **Update package.json**: Modify version before building
3. **Build**: Run `make build` with the updated version
4. **Check if version exists**: See if this exact version is already on
npm
5. **Publish or promote**:
   - New version → Publish normally
- Existing version + tag push → Update dist-tag to `latest` (promotion)
   - Existing version + main push → Skip (already published)

### Handling Version Promotion

If a tagged release (e.g., `v0.3.0`) is pushed but `0.3.0` already
exists on npm (maybe published as a pre-release or from a previous run),
the workflow will **update the dist-tag** to `latest` instead of
failing. This allows promoting versions without re-uploading the
tarball.

Every commit to main now gets a unique, installable pre-release version!
🎉

_Generated with `cmux`_

Author: kylecarbs

.github/workflows/publish-npm.yml

@@ -29,22 +29,79 @@ jobs:
         with:
           registry-url: 'https://registry.npmjs.org'
 
+      - name: Generate unique version from git
+        id: version
+        run: |
+          # Get base version from package.json
+          BASE_VERSION=$(node -p "require('./package.json').version")
+          
+          # Generate git describe version
+          GIT_DESCRIBE=$(git describe --tags --always --dirty 2>/dev/null || echo "unknown")
+          
+          if [[ $GITHUB_REF == refs/tags/* ]]; then
+            # For tags, use the base version as-is (stable release)
+            NPM_VERSION="${BASE_VERSION}"
+            NPM_TAG="latest"
+            echo "Publishing stable release: ${NPM_VERSION}"
+          else
+            # For main branch, create a pre-release version using git describe
+            # Format: 0.3.0-next.5.g1a2b3c4 (base-next.commits.hash)
+            GIT_COMMIT=$(git rev-parse --short HEAD)
+            COMMITS_SINCE_TAG=$(git rev-list --count HEAD ^$(git describe --tags --abbrev=0 2>/dev/null || echo HEAD) 2>/dev/null || echo "0")
+            NPM_VERSION="${BASE_VERSION}-next.${COMMITS_SINCE_TAG}.g${GIT_COMMIT}"
+            NPM_TAG="next"
+            echo "Publishing pre-release: ${NPM_VERSION}"
+          fi
+          
+          echo "version=${NPM_VERSION}" >> $GITHUB_OUTPUT
+          echo "tag=${NPM_TAG}" >> $GITHUB_OUTPUT
+          
+          # Update package.json with the new version
+          node -e "const fs = require('fs'); const pkg = JSON.parse(fs.readFileSync('package.json')); pkg.version = '${NPM_VERSION}'; fs.writeFileSync('package.json', JSON.stringify(pkg, null, 2) + '\n');"
+          
+          echo "Updated package.json to version ${NPM_VERSION}"
+
       - name: Generate version file
         run: ./scripts/generate-version.sh
 
       - name: Build application
         run: make build
 
-      - name: Determine NPM tag
-        id: npm-tag
+      - name: Check if version exists
+        id: check-exists
         run: |
-          if [[ $GITHUB_REF == refs/tags/* ]]; then
-            echo "tag=latest" >> $GITHUB_OUTPUT
-            echo "Publishing as 'latest' tag (stable release)"
+          PACKAGE_NAME=$(node -p "require('./package.json').name")
+          VERSION="${{ steps.version.outputs.version }}"
+          
+          if npm view "${PACKAGE_NAME}@${VERSION}" version &>/dev/null; then
+            echo "exists=true" >> $GITHUB_OUTPUT
+            echo "Version ${VERSION} already exists on npm"
           else
-            echo "tag=next" >> $GITHUB_OUTPUT
-            echo "Publishing as 'next' tag (pre-release from main)"
+            echo "exists=false" >> $GITHUB_OUTPUT
+            echo "Version ${VERSION} does not exist, will publish"
           fi
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
 
       - name: Publish to NPM
-        run: npm publish --tag ${{ steps.npm-tag.outputs.tag }}
+        if: steps.check-exists.outputs.exists == 'false'
+        run: npm publish --tag ${{ steps.version.outputs.tag }}
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
+
+      - name: Update dist-tag (version already exists)
+        if: steps.check-exists.outputs.exists == 'true' && github.ref_type == 'tag'
+        run: |
+          PACKAGE_NAME=$(node -p "require('./package.json').name")
+          VERSION="${{ steps.version.outputs.version }}"
+          TAG="${{ steps.version.outputs.tag }}"
+          
+          echo "Version ${VERSION} already published, updating dist-tag to ${TAG}"
+          npm dist-tag add "${PACKAGE_NAME}@${VERSION}" "${TAG}"
+        env:
+          NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
+
+      - name: Skip (pre-release already exists)
+        if: steps.check-exists.outputs.exists == 'true' && github.ref_type != 'tag'
+        run: |
+          echo "⏭️ Pre-release version already exists, skipping"