refactor(kanban): extract spawnable probe into spawnable_probe (kanban_db.py god-file slice R4)

[andrexibiza]

Summary

kanban_db.py god-file slice R4: extract the spawnable probe (window 8147–8201, 55 lines) into hermes_cli/spawnable_probe.py. Part of the repo-wide large-file decomposition (tracker #78647).

What changed and why

• has_spawnable_ready + has_spawnable_review moved byte-verbatim (golden sha256 0728c1fe… / sha1 e869e064… verified)
• Collision-free vs all 9 open kanban PRs; refactor(kanban): extract workspace/worker mixins from kanban_db.py (shard s4) #79616's workspace territory untouched
• Consumers (gateway/kanban_watchers.py:1310/1312, kanban.py:2663) resolve via re-export
• Double-blind: 2 analysts → consensus → implementer → 2 blind re-reviewers (both APPROVED, zero findings)

Testing

• New test_spawnable_probe.py (165 lines) — identity + aggressive cases
• ruff clean · git diff --check clean · LF-only · DCO signed

Coordination / interlock

• Epic: Part of [COMPLETE] Large-file decomposition: 20/20 done #78647 · Target: Part of Shard hermes_cli/kanban_db.py (god-file decomposition) #78632
• Sibling PRs: the 4 other kanban_db slices — disjoint windows
• Dedup: no existing PR extracts the spawnable probe

Part of #78647
Part of #78632