Add nidusctl admin CLI for managing calendar/address-book shares

- tools/nidusctl: subcommands `calendar {share,unshare,shares}` and
  `addressbook {share,unshare,shares}`, thin wrapper around internal/db.
  Warns (non-fatal) if owner/user isn't in config.yaml.
- internal/db: add SharesOfAddressBook (owner-perspective query, mirrors
  SharesOfCalendar) needed by the CLI.
- internal/db: db.Open now creates the parent data directory itself, so
  the CLI works standalone without requiring the server to have run first.
- Update Makefile (build bin/nidusctl, new 'nidusctl' target), README and
  copilot-instructions with usage docs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/db/db.go

@@ -8,6 +8,8 @@ package db
 import (
 	"database/sql"
 	"fmt"
+	"os"
+	"path/filepath"
 
 	_ "modernc.org/sqlite"
 )
@@ -19,8 +21,15 @@ type DB struct {
 }
 
 // Open opens (creating if necessary) the SQLite database at path and runs
-// schema migrations.
+// schema migrations. The parent directory of path is created if it
+// doesn't already exist.
 func Open(path string) (*DB, error) {
+	if dir := filepath.Dir(path); dir != "" && dir != "." {
+		if err := os.MkdirAll(dir, 0o755); err != nil {
+			return nil, fmt.Errorf("creating database directory %q: %w", dir, err)
+		}
+	}
+
 	conn, err := sql.Open("sqlite", path)
 	if err != nil {
 		return nil, fmt.Errorf("opening database %q: %w", path, err)