tsk remote: drop add/remove/list, validate set-default
Per follow-up: `tsk remote add` and `tsk remote remove` were thin
wrappers around git's own commands; users can run `git remote add`
themselves and follow with `tsk git-setup -r <name>` when they want
the tsk refspecs configured. `tsk remote list` was equivalent to
`git remote`. All three are gone.
What's left is the tsk-specific state:
- `tsk remote default` — print the persisted default
- `tsk remote set-default <n>` — persist `<n>` as the default;
validates against `git remote` first so we can't be pointed at a
remote the host repo doesn't know about.
Workspace::git_remotes survives as the validation helper; the
git_remote_add / git_remote_remove wrappers are gone.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>