fix(next): distinguish filtered empty views from completed queues

[shogo-nakai]

A scoped next or backlog view could report “Nothing to do” or surface a synthetic rescan action even while unrelated execution work remained. Empty views now identify the active filter, and next reports the unfiltered queue count with a continuation command. Workflow actions remain available in the global queue and appear in scoped views only when they match the requested scope.

Validation:

• Added regressions for scope, status, persistent cluster focus, actual queue-builder workflow actions, and filtered backlog rendering.
• Full command suite: 1,750 passed.
• Verified against a copied real project state: an absent scope reports zero matching items and 505 items in the full queue.
• Ruff and diff checks pass.