feat: add dag_engine app — CLI + web frontend for DAG execution (0007e)

Full DAG engine app with CLI subcommands (run, list, status, validate, server)
and React/Mantine web frontend. Uses net/http + embedded Vite build. SQLite
store for run history. Scheduler with cron_ticker for automated execution.
Compatible with existing dagu YAML format.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/dag_engine/config.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"flag"
+	"os"
+	"path/filepath"
+)
+
+// Config holds the runtime configuration for the DAG engine.
+type Config struct {
+	Port          int
+	DagsDir       string
+	DBPath        string
+	AutoScheduler bool
+}
+
+// DefaultConfig returns sensible defaults.
+func DefaultConfig() Config {
+	home, _ := os.UserHomeDir()
+	return Config{
+		Port:    8090,
+		DagsDir: filepath.Join(home, "dagu", "dags"),
+		DBPath:  "dag_engine.db",
+	}
+}
+
+// ParseFlags populates config from CLI flags for the "server" subcommand.
+func (c *Config) ParseFlags(fs *flag.FlagSet, args []string) error {
+	fs.IntVar(&c.Port, "port", c.Port, "HTTP server port")
+	fs.StringVar(&c.DagsDir, "dags-dir", c.DagsDir, "directory containing DAG YAML files")
+	fs.StringVar(&c.DBPath, "db", c.DBPath, "path to SQLite database")
+	fs.BoolVar(&c.AutoScheduler, "scheduler", c.AutoScheduler, "auto-start cron scheduler")
+	return fs.Parse(args)
+}