fix(subagent): inherit parent policy overrides in continuable children

A continuable background child (the default backgroundMode for both
delegation tools) never received the parent session's explicit
sandbox/approval overrides: materialization applied only child
composition, so a danger-full-access parent produced workspace-write
children whose every out-of-workspace operation raised an approval
prompt.

Move the one-shot driver's capture/append pair into the shared
child-agent module (captureDelegatedPolicyOverrides /
appendDelegatedPolicyOverrides) and call it from both paths:
startContinuable captures before its first await, only fresh
materialization appends the source-tagged events (after any fork seed),
and a cold resume replays the persisted delegation events instead of
re-capturing the parent.

Adds the continuable inheritance unit suite, the ACP snapshot scenario
subagent-continuable-inheritance (fails without the fix), the
continuable policy-inheritance Agent Note, and the seam-level README
contract, with bilingual counterparts.

Fixes #1692

.agents/notes/implemented/feature/2026-08-10-continuable-subagent-policy-inheritance.md

@@ -0,0 +1,29 @@
+# Agent Note: Continuable subagent policy inheritance — the durable child log owns the delegation-time snapshot
+
+Status: implemented
+
+English | [中文](2026-08-10-continuable-subagent-policy-inheritance.zh.md)
+
+## Problem
+
+The one-shot in-process driver has seeded parent sandbox/approval overrides into its children since the [in-process policy-inheritance decision](2026-07-25-subagent-policy-inheritance.md), but the continuable path never did: `SubagentContinuationManager` materialization applied only child composition and the activation setup registry. The default bundle wires both delegation tools as `backgroundMode: continuable`, so in a default deployment every background child silently fell back to deployment defaults — a parent switched to `danger-full-access` produced children stuck at `workspace-write` whose every out-of-workspace operation raised an approval prompt, and a parent's unattended `'never'` approval stance reverted to prompting ([dsh-external/issues#334](https://github.com/dsh-external/issues/issues/334)).
+
+## Decision
+
+The capture/append pair moved from the one-shot driver into the seam's shared child-agent module (`dsh-subagent/src/child-agent.ts`), the declared one home for shared child composition: `captureDelegatedPolicyOverrides(parent)` snapshots `sandboxPolicy.overrideOf(parent.session)` and `approval.overrideOf(parent.session)` through optional `ctx.get`, and `appendDelegatedPolicyOverrides(childSession, overrides)` appends the `source: 'delegation'` events. The one-shot driver and the continuation manager both call them, so the two paths cannot drift.
+
+`startContinuable` captures before its first await (`prepareContinuable`), the same "a later parent switch belongs to the parent's future" boundary as one-shot. The snapshot travels in `MaterializeInputs.create`, so only fresh materialization appends the events during unpublished setup, after any fork seed. A cold resume passes no `create` inputs and appends nothing: the persisted child log already carries the delegation events, and replaying the log IS the state. The durable child log — not the current Activation, not the resuming parent — owns the child's effective policy, so a parent switch between residency epochs never retroactively changes a durable child.
+
+## Alternatives considered
+
+- **An activation-setup-registry contribution** (`registerContinuableSetup`) — rejected: a contribution receives only the child context, so it cannot capture the parent's overrides at the delegation boundary; the registry applies on cold resume as well as fresh creation, which would re-append or re-capture; and nothing ties a contribution's capture to the start call's synchronous prefix, so the pre-await capture guarantee would be lost.
+- **Re-capturing the parent's overrides at cold resume** — rejected: a resumed child would silently change policy with the parent's later switches, breaking the snapshot-at-delegation semantic and making effective policy depend on resume timing instead of the child's own log. A parent that wants a resumed child under new policy re-delegates.
+- **Importing the one-shot driver's inline logic from the continuation manager** — rejected: the Service Definition package cannot depend on its own provider package, and duplicating the capture/append pair in `continuation.ts` invites drift; `child-agent.ts` already holds every other shared composition step.
+- **Seeding the events into the descriptor seed turn** — rejected: the capture value is not known when the seed is assembled for every caller, and the one-shot precedent already establishes unpublished-setup appends as the ordering that places inherited facts after fork history with `firstLiveSeq` intact.
+
+## Consequences
+
+- Default-bundle background delegation (`backgroundMode: continuable`) now inherits a parent's explicit sandbox and approval overrides; compositions without either policy service behave unchanged.
+- `dsh-subagent` gains optional peer types on `dsh-sandbox-policy` and `dsh-user-approval` (the `ctx.get` pattern the one-shot driver used); `dsh-subagent-inprocess` keeps its optional peers but delegates to the shared helpers.
+- The continuable suite (`packages/subagent/subagent/tests/continuation-inheritance.spec.ts`) pins fresh-start seeding, pre-await capture, default omission, cold-resume snapshot stability, and fork-seed precedence; the ACP snapshot scenario `subagent-continuable-inheritance` pins the child's delegation event and read-only runtime context through the assembled app and fails when the capture is removed.
+- Out-of-process providers (`acp`, `dsh-sdk`, `claude-code`, `codex`) support no continuable children (`prepareContinuable` absent), and their one-shot children keep their own deployment policy (`inheritsParentContext = false`); cross-process policy propagation remains out of scope.