Remove legacy .tsk/ migration and tighten dead-code allows
The pre-rewrite file-based workspace lived under `<repo>/.tsk/`. Its
auto-migration into `<git-dir>/tsk/` has been in place long enough that
no live workspace still needs it; drop the migration block from
Workspace::init and its accompanying test. Update workspace and queue
module docs to describe the current ref-only storage.
Replace the blanket #![allow(dead_code)] on workspace.rs with targeted
field-level allows on Task.id/Task.stable/InboxItem.stable — those are
exposed API even though nothing reads them yet, but the rest of the
file is now warning-clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>