feat: project init with config package and root command

Go module, cobra root command, config resolution (CTASK_ROOT, CTASK_AGENT, EnvVars) with tests.

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

main.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"os"
+
+	"github.com/warrenronsiek/ctask/cmd"
+)
+
+func main() {
+	if err := cmd.Execute(); err != nil {
+		os.Exit(1)
+	}
+}