tsk remote: manage non-origin remotes + per-clone default
`tsk remote` subcommands:
- `list` — wraps `git remote`
- `default` — print the active default
- `set-default <n>` — persist the default to <git-dir>/tsk/remote
- `add <name> <url>` — `git remote add` + configure tsk refspecs
- `remove <name>` — `git remote remove`
The default remote is a per-clone selector parallel to `namespace`
and `queue`, stored at `<git-dir>/tsk/remote`. Falls back to
`origin` when absent. `tsk git-push`, `tsk git-pull`, `tsk
git-setup`, and the auto-push paths after assign/accept/reject all
consult it when no explicit `<remote>` / `-R <remote>` is given.
ARCHITECTURE.md and AGENTS.md updated to mention the new selector
and the new commands.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>