feat(T1.2): implement structured logging and Prometheus metrics

- Add internal/logging package with zap-based structured JSON logging
- Support development (colored) and production (JSON) modes via ENVIRONMENT env var
- Add logging helpers: Info(), Error(), Warn(), Debug(), Fatal()
- Add field helpers: String(), Int(), Int64(), Err()
- Add internal/metrics package with 16 comprehensive Prometheus metrics
- Track workflows: starts, completions, duration by type/status
- Track activities: starts, completions, duration, retries by type
- Track LLM calls: total calls and latency by model
- Track git operations: total and duration by operation type
- Track judge decisions: decisions by type
- Track Temporal errors: connection errors by type
- Track cache efficiency: hits and misses by cache type
- Track tasks in progress: gauge metric by task type
- Metrics exported on /metrics endpoint (Prometheus text format)
- Integrate structured logging in cmd/worker and cmd/starter
- Replace all log.Printf/log.Fatalf with structured logging
- Add /metrics endpoint to health check server
- 8/8 logging tests passing, 13/13 metrics tests passing
- All verification criteria met

Dependencies added:
- go.uber.org/zap v1.28.0 (structured logging)
- github.com/prometheus/client_golang v1.24.1 (metrics export)

Closes T1.2

internal/logging/logger.go

@@ -0,0 +1,97 @@
+package logging
+
+import (
+	"os"
+
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
+)
+
+var logger *zap.Logger
+
+// InitLogger initializes the global logger
+func InitLogger() error {
+	var config zap.Config
+
+	// Use pretty config in development, JSON in production
+	if os.Getenv("ENVIRONMENT") == "production" {
+		config = zap.NewProductionConfig()
+	} else {
+		config = zap.NewDevelopmentConfig()
+		config.EncoderConfig.EncodeLevel = zapcore.CapitalColorLevelEncoder
+	}
+
+	var err error
+	logger, err = config.Build()
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// GetLogger returns the global logger
+func GetLogger() *zap.Logger {
+	if logger == nil {
+		logger, _ = zap.NewProduction()
+	}
+	return logger
+}
+
+// Info logs an info message
+func Info(message string, fields ...zap.Field) {
+	GetLogger().Info(message, fields...)
+}
+
+// Error logs an error message
+func Error(message string, fields ...zap.Field) {
+	GetLogger().Error(message, fields...)
+}
+
+// Warn logs a warning message
+func Warn(message string, fields ...zap.Field) {
+	GetLogger().Warn(message, fields...)
+}
+
+// Debug logs a debug message
+func Debug(message string, fields ...zap.Field) {
+	GetLogger().Debug(message, fields...)
+}
+
+// Fatal logs a fatal message and exits
+func Fatal(message string, fields ...zap.Field) {
+	GetLogger().Fatal(message, fields...)
+}
+
+// Sync flushes any buffered log entries
+func Sync() error {
+	if logger != nil {
+		return logger.Sync()
+	}
+	return nil
+}
+
+// With returns a child logger with additional fields
+func With(fields ...zap.Field) *zap.Logger {
+	return GetLogger().With(fields...)
+}
+
+// String is a helper for creating a string field
+func String(key, value string) zap.Field {
+	return zap.String(key, value)
+}
+
+// Int is a helper for creating an int field
+func Int(key string, value int) zap.Field {
+	return zap.Int(key, value)
+}
+
+// Int64 is a helper for creating an int64 field
+func Int64(key string, value int64) zap.Field {
+	return zap.Int64(key, value)
+}
+
+// Error field helper
+func Err(err error) zap.Field {
+	return zap.Error(err)
+}