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.

cmd/store.go

@@ -0,0 +1,13 @@
+package cmd
+
+import (
+	"github.com/lerko/nib/internal/db"
+)
+
+func openStore() (*db.Store, error) {
+	path, err := db.DefaultPath()
+	if err != nil {
+		return nil, err
+	}
+	return db.Open(path)
+}