Show and manage labels on issues and pull requests #84

tg repo edit --add-label/--remove-label manages a repository's label set, but the labels can't be seen or applied anywhere else:

• tg repo view --json doesn't list the repository's labels.
• tg issue list/view --json and tg pr list/view --json don't include the labels on each item.
• No way to add or removes a label on an issue or pull request.

Suggested #

• labels (name, color, definition URI) in tg repo view --json.
• A labels array in the --json output of issue list/view and pr list/view, plus a plain-text column in the default output.
• --label NAME filters on issue list and pr list.
• --add-label NAME and --remove-label NAME on tg issue edit and tg pr edit, resolving label list against the repository’s org.tangled.label.definition, then update the labels on its org.tangled.track.ticket record through the repository’s knot, authenticated as the active account.