docs: review brief schema 8 fields (action, test adjacency, slices, dependency decisions)

[BartWaardenburg]

Documents the review-brief additions that land with the fallow release shipping review-brief schema 8:

• Decision surface: the dependency category now covers added entries and major bumps, one batched decision per manifest per kind.
• JSON envelope: deltas carry dependency_added and dependency_major_bumped; partition carries independent_slices.
• Walkthrough guide: direction units carry test_adjacency; agent_schema publishes action_vocabulary and concern_vocabulary.
• Judgment JSON: optional action label, the invalid-action rejection, and action echoed on accepted judgments.

Hold until that fallow release is published.