Fix non-constant format string lint errors in proxy handler (#3415)

## Summary

Fixes the failing `lint` GitHub Actions workflow (job ID `70548695541`)
caused by `go vet` detecting non-constant format strings.

## Changes

- `internal/proxy/handler.go:144`: Changed `logHandler.Printf(errMsg)` →
`logHandler.Print(errMsg)` (uses the non-format variant)
- `internal/proxy/handler.go:145`: Changed `logger.LogError("proxy",
errMsg)` → `logger.LogError("proxy", "%s", errMsg)` (wraps variable in
`%s` format specifier)

## Verification

- `make agent-finished` passes (format, build, lint, all tests)

Author: lpcox

internal/proxy/handler.go

@@ -141,8 +141,8 @@ func (h *proxyHandler) handleWithDIFC(w http.ResponseWriter, r *http.Request, pa
 
 	if !s.guardInitialized {
 		errMsg := "returning 503: proxy enforcement not configured (no --policy flag provided)"
-		logHandler.Printf(errMsg)
-		logger.LogError("proxy", errMsg)
+		logHandler.Print(errMsg)
+		logger.LogError("proxy", "%s", errMsg)
 		http.Error(w, "proxy enforcement not configured", http.StatusServiceUnavailable)
 		return
 	}