feat(cli): add nib add + nib ls commands

Default command delegation: `nib "..."` routes to `nib add`. Capture
bar parses grammar, creates entities. `nib ls` lists with date
grouping, tag filter, 48h default window. Display glyphs for all
entity types.

main.go

@@ -1,7 +1,15 @@
 package main
 
-import "fmt"
+import (
+	"fmt"
+	"os"
+
+	"github.com/lerko/nib/cmd"
+)
 
 func main() {
-	fmt.Println("nib")
+	if err := cmd.Execute(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
 }