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/shares.go

@@ -178,6 +178,33 @@ func (d *DB) UnshareAddressBook(owner, bookName, sharedWith string) error {
 	return nil
 }
 
+// SharesOfAddressBook lists everyone owner's address book bookName has
+// been shared with.
+func (d *DB) SharesOfAddressBook(owner, bookName string) ([]AddressBookShare, error) {
+	rows, err := d.conn.Query(`
+		SELECT owner, addressbook_name, shared_with, permission
+		FROM addressbook_shares
+		WHERE owner = ? AND addressbook_name = ?
+		ORDER BY shared_with`,
+		owner, bookName)
+	if err != nil {
+		return nil, fmt.Errorf("listing address book shares: %w", err)
+	}
+	defer rows.Close()
+
+	var shares []AddressBookShare
+	for rows.Next() {
+		var s AddressBookShare
+		var perm string
+		if err := rows.Scan(&s.Owner, &s.AddressBookName, &s.SharedWith, &perm); err != nil {
+			return nil, fmt.Errorf("scanning address book share: %w", err)
+		}
+		s.Permission = Permission(perm)
+		shares = append(shares, s)
+	}
+	return shares, rows.Err()
+}
+
 // AddressBooksSharedWith lists all address books (from any owner) that
 // have been shared with user.
 func (d *DB) AddressBooksSharedWith(user string) ([]AddressBookShare, error) {