Add service orchestration and command

Wire the intake and answer subsystems together into a running
application.

- config: maps YAML file to internal package configuration.
- service: manages lifecycle and graceful shutdown.
- cmd/raven: entry point for flag parsing and signal handling.

cmd/raven/main.go

@@ -0,0 +1,82 @@
+// Command raven monitors an IMAP mailbox for new messages, generates replies
+// using an LLM, and sends them via SMTP. See package service for the main
+// orchestration logic and package config for configuration options.
+package main
+
+import (
+	"context"
+	"flag"
+	"log/slog"
+	"os"
+	"runtime/debug"
+
+	"raven/internal/config"
+	"raven/internal/service"
+)
+
+func main() {
+	var cfgPath = flag.String(
+		"c", "", "path to configuration file (required)",
+	)
+	flag.Parse()
+
+	var log = slog.New(slog.NewTextHandler(
+		os.Stderr,
+		&slog.HandlerOptions{
+			AddSource: true,
+			Level:     slog.LevelDebug,
+		},
+	))
+	if *cfgPath == "" {
+		log.Error("missing config path")
+		os.Exit(1)
+	}
+
+	// Log build info.
+	bi, ok := debug.ReadBuildInfo()
+	if ok {
+		var commit, time string
+		for _, s := range bi.Settings {
+			switch s.Key {
+			case "vcs.revision":
+				commit = s.Value
+			case "vcs.time":
+				time = s.Value
+			}
+		}
+		log.Debug(
+			"raven",
+			slog.Group(
+				"build",
+				slog.String("commit", commit),
+				slog.String("time", time),
+			),
+		)
+	}
+
+	cfg, err := config.Load(*cfgPath)
+	if err != nil {
+		log.Error(
+			"failed to load config",
+			slog.Any("error", err),
+		)
+		os.Exit(1)
+	}
+
+	svc, err := service.New(cfg, log)
+	if err != nil {
+		log.Error(
+			"failed to create service",
+			slog.Any("error", err),
+		)
+		os.Exit(1)
+	}
+
+	if err := svc.Run(context.Background()); err != nil {
+		log.Error(
+			"failed to run",
+			slog.Any("error", err),
+		)
+		os.Exit(1)
+	}
+}