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

cmd/worker/main.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"context"
-	"fmt"
 	"log"
 	"net/http"
 	"os"
@@ -15,14 +14,21 @@ import (
 	"github.com/rockliang/poimen/workflows/action"
 	"github.com/rockliang/poimen/workflows/internal/config"
 	"github.com/rockliang/poimen/workflows/internal/health"
+	"github.com/rockliang/poimen/workflows/internal/logging"
 	"github.com/rockliang/poimen/workflows/statemachine"
 )
 
 func main() {
+	// Initialize structured logging
+	if err := logging.InitLogger(); err != nil {
+		log.Fatalf("failed to initialize logger: %v", err)
+	}
+	defer logging.Sync()
+
 	// Load configuration
 	cfg, err := config.LoadConfig()
 	if err != nil {
-		log.Fatalf("failed to load config: %v", err)
+		logging.Fatal("failed to load config", logging.Err(err))
 	}
 
 	// Connect to Temporal
@@ -31,14 +37,14 @@ func main() {
 		Namespace: cfg.Temporal.Namespace,
 	})
 	if err != nil {
-		log.Fatalf("failed to connect to temporal: %v", err)
+		logging.Fatal("failed to connect to temporal", logging.Err(err))
 	}
 	defer c.Close()
 
 	// Create worker
 	w := worker.New(c, "poimen-taskqueue", worker.Options{})
 	if w == nil {
-		log.Fatalf("failed to create worker")
+		logging.Fatal("failed to create worker")
 	}
 
 	// Register all workflows
@@ -90,7 +96,7 @@ func main() {
 	// Run worker in a goroutine
 	workerErrChan := make(chan error, 1)
 	go func() {
-		fmt.Println("Starting worker on queue 'poimen-taskqueue'...")
+		logging.Info("starting worker on queue", logging.String("queue", "poimen-taskqueue"))
 		if err := w.Run(worker.InterruptCh()); err != nil {
 			workerErrChan <- err
 		}
@@ -99,16 +105,17 @@ func main() {
 	// Wait for either worker error or signal
 	select {
 	case err := <-workerErrChan:
-		log.Fatalf("worker failed: %v", err)
+		logging.Fatal("worker failed", logging.Err(err))
 	case sig := <-sigChan:
-		log.Printf("received signal: %v, shutting down gracefully", sig)
+		logging.Info("received signal", logging.String("signal", sig.String()))
 		w.Stop()
 
 		// Shutdown health check server
 		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 		defer cancel()
 		if err := healthServer.Shutdown(ctx); err != nil {
-			log.Printf("health check server shutdown error: %v", err)
+			logging.Warn("health check server shutdown error", logging.Err(err))
 		}
+		logging.Info("worker shutdown complete")
 	}
 }