fix(mode): the default mode hides the exit binding from the Code Mode SDK too

Review finding, valid — the previous SDK fix covered only the
non-default branch: in the default mode under Code Mode the wire filter
dropped exit_plan_mode but the registry-rendered tools:sdk section
still advertised its binding, offering default-mode agents a call that
can only error and breaking the byte-identical claim (a no-dsh-mode
deployment's registry never saw the tool, so its SDK never listed it).

The SDK re-render extracts to one helper both branches share: the
non-default branch passes the mode's visibility rule, the default
branch hides exactly the exit binding. The pinning test now compares
the default-mode SDK byte-for-byte against a bare deployment without
dsh-mode — the strongest form of the invariant the RFC states.

docs/rfc/implemented/feature/2026-07-07-plan-mode.md

@@ -94,7 +94,7 @@ A contained `session/event` listener ([defensive patterns](../../../defensive-pa
 
 A `system-prompt/assemble` waterfall listener reads the calling agent's mode (the `AssembleContext` carries `agent`) and, in a non-default mode, filters `assembly.tools` down to the mode's allowlist and appends the mode's guidance section. The loop already renders per step and logs the result: entering or leaving a mode surfaces on the next step as a `request/header-delta` — or as the full `request/header` fallback snapshot when the change is inexpressible in the delta encoding (adding `exit_plan_mode` resorts the canonical tool list, and a pure reordering has no delta form) — so every mode transition is an attributable log fact. The section is static per mode and the plan itself stays in the conversation (messages and tool args, already in context), so a mode does not add per-step prompt churn — re-injecting plan state into every request ([Prior art](#prior-art)'s compaction-survival hack) is unnecessary and would only burn prefix cache.
 
-The guidance section is an ordinary registered section, `{ name: 'mode:policy', order: 50, text: context => … }` — order 50 sits after the persona (0) and before tool guidance (100–199); it resolves to the folded mode's configured text and to `''` (dropped at render) for the default mode or an agent-less assembly. The tool filter wraps with `prepend: true`: it awaits `next()` and filters the RETURNED assembly's `tools`, so additions made anywhere inside its wrap — including every append-registered listener's post-`next()` mutation, regardless of load order — are covered. `run_code` survives the filter in every mode: under the registry's Code Mode it is the only wire tool (filtering it would strip the model of everything, the exit included), and it is a transport, not a capability — each bridged sub-call re-enters the hard gate individually. Code Mode's soft surface is the `tools:sdk` section rather than the wire schemas, and section text resolves in assemble's base, so the same wrapper re-renders that section under the mode's visibility rule — the prompt documents exactly the callable bindings, never one the gate would deny. The filter enforces one rule in every mode: `exit_plan_mode` is visible IFF the agent's folded mode is `plan` — which is also what keeps a default-mode assembly byte-identical to a no-`dsh-mode` deployment even though the tool is always registered. In a non-default mode it additionally intersects with the mode's allowlist.
+The guidance section is an ordinary registered section, `{ name: 'mode:policy', order: 50, text: context => … }` — order 50 sits after the persona (0) and before tool guidance (100–199); it resolves to the folded mode's configured text and to `''` (dropped at render) for the default mode or an agent-less assembly. The tool filter wraps with `prepend: true`: it awaits `next()` and filters the RETURNED assembly's `tools`, so additions made anywhere inside its wrap — including every append-registered listener's post-`next()` mutation, regardless of load order — are covered. `run_code` survives the filter in every mode: under the registry's Code Mode it is the only wire tool (filtering it would strip the model of everything, the exit included), and it is a transport, not a capability — each bridged sub-call re-enters the hard gate individually. Code Mode's soft surface is the `tools:sdk` section rather than the wire schemas, and section text resolves in assemble's base, so the same wrapper re-renders that section under the mode's visibility rule — the prompt documents exactly the callable bindings, never one the gate would deny. The default mode re-renders it too, hiding only the exit binding: that keeps a default-mode Code Mode assembly byte-identical to a deployment that never loaded `dsh-mode` (whose registry never saw the tool), instead of advertising a binding that can only error. The filter enforces one rule in every mode: `exit_plan_mode` is visible IFF the agent's folded mode is `plan` — which is also what keeps a default-mode assembly byte-identical to a no-`dsh-mode` deployment even though the tool is always registered. In a non-default mode it additionally intersects with the mode's allowlist.
 
 ### The hard layer: the gate