Adds the Detailed design block: the mode/set vocabulary (no provenance
field — cause is log-adjacent; bare-string mode names), ModeConfig with an
explicit resolve step, the cursor-cached fold + ctx.modes service with
turn/start|step/end pending flush, the computed mode:policy section (order
50) + post-next() assembly filter (exit_plan_mode visible IFF plan mode —
what keeps default-mode assemblies byte-identical), the fold-only gate with
its ask on exit_plan_mode (no dsh-approval dependency — the registry
routes), the exit tool contract and render intent, dependency edges and the
stdio/ACP surfaces, the input.json setMode op + plan-mode/plan-mode-reject
scenarios (mode set before turn 1 → the initial header snapshot is already
in plan shape; the widening delta appears at exit), and the mechanical tail.
Consistency fixes: the plan allowlist names real tools (no grep tool
exists); the re-widen risk now states the post-next() mechanism instead of
a listener-order convention.
kingwl
db9217a481