Refactor client initialization for code intel upload

### Problem

The `src code-intel upload` command does not respect the `SRC_PROXY` environment variable, preventing uploads through corporate proxies. This also means standard `HTTP_PROXY`/`HTTPS_PROXY` environment variables are not honored.

**Customer Impact**: Dropbox reported they can successfully upload code intelligence indexes from local machines, but uploads fail from their CI system which requires outbound proxy configuration. The `SRC_PROXY` environment variable works for `src search` and other commands, but not for `src code-intel upload`.

### Root Cause

In `cmd/src/code_intel_upload.go:79-82`, the API client was constructed directly with `api.NewClient()`, passing only `Out` and `Flags`. This bypassed the global configuration object which contains:
- Endpoint
- Access token  
- Additional headers
- **Proxy URL and proxy path** (parsed from `SRC_PROXY` environment variable)

Other commands use the `cfg.apiClient()` helper method which properly includes all configuration.

### Fix

Replace the manual `api.NewClient()` construction with `cfg.apiClient()`, ensuring the code-intel upload command uses the same fully-configured client as all other commands.

Author: natehessler

cmd/src/code_intel_upload.go

@@ -76,10 +76,7 @@ func handleCodeIntelUpload(args []string) error {
 		return handleUploadError(cfg.AccessToken, err)
 	}
 
-	client := api.NewClient(api.ClientOpts{
-		Out:   io.Discard,
-		Flags: codeintelUploadFlags.apiFlags,
-	})
+	client := cfg.apiClient(codeintelUploadFlags.apiFlags, io.Discard)
 
 	uploadOptions := codeintelUploadOptions(out)
 	var uploadID int