fix: align Docker Go version with go.mod and optimize logger (#596)

- Update Dockerfile to use golang:1.25-alpine to match go.mod (go 1.25.7)
- Optimize logger by avoiding string concatenation in file writes
- Add explicit empty string assignment for fieldStr when no fields

These changes improve build consistency and reduce memory allocations
in the hot logging path, which is important for the project's goal
of running on resource-constrained devices (<10MB RAM).

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,7 +1,7 @@
 # ============================================================
 # Stage 1: Build the picoclaw binary
 # ============================================================
-FROM golang:1.26.0-alpine AS builder
+FROM golang:1.25-alpine AS builder
 
 RUN apk add --no-cache git make
 

pkg/logger/logger.go

@@ -119,13 +119,15 @@ func logMessage(level LogLevel, component string, message string, fields map[str
 	if logger.file != nil {
 		jsonData, err := json.Marshal(entry)
 		if err == nil {
-			logger.file.WriteString(string(jsonData) + "\n")
+			logger.file.Write(append(jsonData, '\n'))
 		}
 	}
 
 	var fieldStr string
 	if len(fields) > 0 {
 		fieldStr = " " + formatFields(fields)
+	} else {
+		fieldStr = ""
 	}
 
 	logLine := fmt.Sprintf("[%s] [%s]%s %s%s",