Move user-local namespace and queue selectors into .git/tsk/
The .tsk/ directory was tracked by git, so switching namespace or queue
showed up as a modified file in every clone — and the per-clone choice
leaked into the shared history. State now lives in <git-dir>/tsk/, which
git doesn't track by definition. tsk init is idempotent and migrates any
existing .tsk/ namespace and queue files into the new location, then
removes the legacy directory so it stops appearing in git status.
Drops the now-redundant util module (the old .tsk-discovery walk is gone).
Tests: 2 new workspace tests cover (1) init creating nothing in the
working tree and (2) the legacy migration path.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>