The deliverable is plan mode — retitle and reorder the proposal opener so
the feature leads and the logged session-mode state is its implementation
property. Fix the Codex citation in the boolean-planMode alternative: Codex
has plan mode (/plan); its ACP mode list carries its approval presets — two
different surfaces. Durable vocabulary (mode/set, ctx.modes, dsh-mode) stays
string-shaped so a second mode never renames logged event types.