fix(search): cmd panel opens a fresh search window per invocation
Typing `#tagA` then `#tagB` in the cmd panel previously focused the
existing search window without updating its results. The cmd panel
opened search via `api.window.open` with `key:'search-home'`, which
hits the keepLive-reuse short-circuit in tile:window:open: the new
URL is dropped on the floor and the renderer keeps showing the old
query.
Fix: drop the `key` from `openSearch` (and from the `cmd: search`
command in features/search/background.js). With no key, the
keepLive-reuse path doesn't fire and each invocation creates a
fresh window — so users can compare tag results side-by-side. The
manifest's tile-level `key` only matters at session-restore time,
where descriptors carry whatever key was passed at open time, so
restored windows stay distinct too.
Regression test in cmd-hashtag-search.spec.ts: opens cmd, fires
openSearch with `#alpha`, then again with `#beta`, and asserts that
two separate search windows exist with their respective queries.