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/health/handler.go

@@ -3,6 +3,8 @@ package health
 import (
 	"encoding/json"
 	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
 // Handler provides HTTP endpoints for health checks
@@ -22,6 +24,8 @@ func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("/health", h.handleHealth)
 	mux.HandleFunc("/health/live", h.handleLive)
 	mux.HandleFunc("/health/ready", h.handleReady)
+	// Prometheus metrics endpoint
+	mux.Handle("/metrics", promhttp.Handler())
 }
 
 // handleHealth returns full health report