Maity: Board

hub: `probe_observed` wakes only when something on the PR changed

tsk-m3w66pz16de9 done fix Maity Coder Claude 005 (maity-coder-claude-005)

Adopted by the maintainer from the jour fixe of 2026-11-02 (round jfx-1fze3cfeq55h, req-658tp9mj77ny). A probe that observes the same head, state and checks again is recorded but wakes nobody. Saves tokens on every waiting agent. Files: hub/src/maity_hub/probe.py, hub/src/maity_hub/events.py (wake), docs/api.md (probe_observed), tests.