subagent: capture overrides at delegation; stamp ahead of prompt vetoes

Review fixes (ds-review-bot on #623):

- Capture-at-delegation: the driver now reads overrideOf(parent.session)
  for both knobs synchronously before its first await, and the prompt-submit
  listener stamps those captured values — a parent switch racing the child's
  asynchronous creation belongs to the parent's future, not the child. The
  inheritOverride(parent, child) service method is split into its two halves
  (overrideOf / stampOverride) accordingly.
- Veto safety: the one-shot prompt-submit listener registers with
  prepend: true, so a veto-capable listener (a denying UserPromptSubmit
  hook) cannot close the child's first turn without the durable stamp.

Both regressions are pinned red-first in inheritance.spec.ts: the
delegation-vs-late-switch race (delegate tool flips the caller wider while
the creation transaction is pending) and a blocking prompt-submit listener
(stamp survives a promptless first turn). Service contract tests renamed to
the split API; READMEs and the bilingual Agent Note updated.

.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md

@@ -12,8 +12,8 @@ Session policy overrides are per-session log folds: the effective sandbox mode i
 
 The shared in-process driver (`startInProcessRun` in `packages/subagent/subagent-inprocess`) snapshots the parent's policy overrides at delegation and stamps them onto the child as ordinary log events inside the child's FIRST turn:
 
-- **Read at creation, write at first `agent/prompt-submit`.** The driver installs a one-shot child-scoped `agent/prompt-submit` listener during the creation transaction's setup window. Prompt-submit runs after `turn/start` and before prompt assembly, so the stamped events are turn-enclosed (durable — a bare between-turn event is crash-tail garbage on reload) and visible to the child's very first request (an inherited `'never'` reaches the child's first system prompt). This is the same anchoring the ACP bridge uses for idle preset switches.
-- **Only the override chain is copied, through the canonical write paths.** `SandboxPolicyService.inheritOverride(parent, child)` and `ApprovalService.inheritOverride(parent, child)` each fold the parent's FULL live log (not the fork seed), append via `setSandboxMode`/`setApprovalPolicy` only when the parent has an override the child does not already fold to, and never copy the deployment default — an unswitched parent stamps nothing, so a resumed child keeps following the LIVE default. The driver consumes both services opportunistically (`ctx.get`, type-only imports): compositions without them delegate policy-free, unchanged.
+- **Capture synchronously at delegation, stamp at first `agent/prompt-submit`.** The driver reads `overrideOf(parent.session)` for both knobs BEFORE its first await — the delegation moment is the snapshot point, so a parent switch racing the asynchronous child creation belongs to the parent's future, not the child — and installs a one-shot child-scoped `agent/prompt-submit` listener during the creation transaction's setup window, PREPENDED so a veto-capable listener (a denying UserPromptSubmit hook) cannot close the first turn without the stamp. Prompt-submit runs after `turn/start` and before prompt assembly, so the stamped events are turn-enclosed (durable — a bare between-turn event is crash-tail garbage on reload) and visible to the child's very first request (an inherited `'never'` reaches the child's first system prompt). This is the same anchoring the ACP bridge uses for idle preset switches.
+- **Only the override chain is copied, through the canonical write paths.** `overrideOf(session)` is the fold alone — never the deployment/configured default — so an unswitched parent stamps nothing and a resumed child keeps following the LIVE default; `stampOverride(child, value)` appends via `setSandboxMode`/`setApprovalPolicy` unless the child already folds to the value. The driver consumes both services opportunistically (`ctx.get`, type-only imports): compositions without them delegate policy-free, unchanged.
 - **Fork stale-seed precedence falls out of log order.** The stamped event lands after any switch the seed carried, so the existing last-event-wins fold resolves the child's mode with no new precedence machinery; an equal seed-carried override is deduplicated instead of re-stamped.
 - **Nesting composes by construction.** A grandchild's stamp folds its parent-the-child's log, which already contains the child's stamped (or self-switched) override — the chain collapses one level per delegation, at any depth. One-shot `allowed-once` escalation grants never enter any log, so they can never leak down the chain.
 
@@ -31,7 +31,7 @@ A confined child that hits the wall gets the ordinary denial marker; an escalati
 
 ## Consequences
 
-- A parent's tightened sandbox mode and `'never'` approval stance now bind spawn children, fork children (regardless of seed timing), and grandchildren; the delegation bypass is closed at every depth. Pinned by the real-wall suite in `packages/subagent/subagent-inprocess/tests/inheritance.spec.ts` (a scripted-model child hitting the real `dsh-fs-sandbox` fence through the real `write` tool, asserted on disk state and denial markers) and the `inheritOverride` contract tests in the two service suites.
+- A parent's tightened sandbox mode and `'never'` approval stance now bind spawn children, fork children (regardless of seed timing), and grandchildren; the delegation bypass is closed at every depth. Pinned by the real-wall suite in `packages/subagent/subagent-inprocess/tests/inheritance.spec.ts` (a scripted-model child hitting the real `dsh-fs-sandbox` fence through the real `write` tool, asserted on disk state and denial markers — including the delegation-vs-late-switch race and a veto-capable prompt-submit listener) and the `overrideOf`/`stampOverride` contract tests in the two service suites.
 - The stamped override is the child's own durable record: resume replays it like any switch, and the child may later be switched independently without the driver re-stamping over it (one-shot listener + fold dedup).
 - Accepted limits: a parent switch made while a child is already running does not propagate (snapshot semantics); a child hard-killed before its first `turn/end` loses the stamp on resume (worthless-resume corner, recorded above); out-of-process backends (`subagent-acp`, subprocess children) inherit nothing here — their policy belongs to the child harness's own deployment, the sandbox Agent Note's deferred phase.
 - `dsh-subagent-inprocess` now declares `dsh-sandbox-policy` and `dsh-user-approval` as peers for the `ctx.get` typing; both remain runtime-optional.