feat: add millisecond-precision timestamps to all log entries (#91)

- Add timestamp prefix format [YYYY-MM-DD HH:MM:SS.mmm] to every log entry
- Resolves issue where logs lacked time information, making it impossible to determine when events (like "Unknown event format" errors) occurred
- Update tests to handle new timestamp format by stripping prefixes during validation
- All 27+ tests pass with new format

Implementation:
- Modified logger.go:369-370 to inject timestamp before message
- Updated concurrent_stress_test.go to strip timestamps for format checks

Fixes #81

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Co-authored-by: SWE-Agent.ai <noreply@swe-agent.ai>

codeagent-wrapper/logger.go

@@ -366,7 +366,8 @@ func (l *Logger) run() {
 	defer ticker.Stop()
 
 	writeEntry := func(entry logEntry) {
-		fmt.Fprintf(l.writer, "%s\n", entry.msg)
+		timestamp := time.Now().Format("2006-01-02 15:04:05.000")
+		fmt.Fprintf(l.writer, "[%s] %s\n", timestamp, entry.msg)
 
 		// Cache error/warn entries in memory for fast extraction
 		if entry.isError {