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

docs/subsystems/subagent.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/subsystems/subagent.md
-subagent.md: 99c54e0696c9c3585c50285ecb69b14c90d83c11
-subagent.zh.md: c5a79247a81f5174662f2b5d1ed2d3c20cf6c672
+subagent.md: b554bb20180b8784e38ac14fb83769eb354b53ae
+subagent.zh.md: 14e2b9b3e571c97384ccf560ce23edbeda62a305

docs/subsystems/subagent.md

@@ -613,7 +613,7 @@ async start(name: string, request: SubagentStartRequest): Promise<SubagentRun>
 
 Types: [Agent](core.md) · [ContentBlock](llm-streaming.md) · [MessageId](llm-streaming.md) · [SessionId](core.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:167`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:169`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagent-events"></a>
 
@@ -639,7 +639,7 @@ A published child settled. Scope-filtered dispatch uses the same delegating pare
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:162`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:164`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-added--emit"></a>
 
@@ -656,7 +656,7 @@ A provider became resolvable in the registry.
 'subagent/provider-added'(provider: SubagentProvider): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:136`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:138`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-removed--emit"></a>
 
@@ -673,7 +673,7 @@ A provider left the registry. Accepted runs remain holder-owned.
 'subagent/provider-removed'(name: string): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:142`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:144`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentstart--emit"></a>
 
@@ -697,5 +697,5 @@ A provider established a published child. For in-process providers, `ctx.agents.
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:153`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:155`](../../packages/subagent/subagent/src/index.ts)
 <!-- END GENERATED cordis-surface -->

docs/subsystems/subagent.zh.md

@@ -615,7 +615,7 @@ async start(name: string, request: SubagentStartRequest): Promise<SubagentRun>
 
 Types: [Agent](core.md) · [ContentBlock](llm-streaming.md) · [MessageId](llm-streaming.md) · [SessionId](core.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:167`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:169`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagent-events"></a>
 
@@ -641,7 +641,7 @@ A published child settled. Scope-filtered dispatch uses the same delegating pare
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:162`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:164`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-added--emit"></a>
 
@@ -658,7 +658,7 @@ A provider became resolvable in the registry.
 'subagent/provider-added'(provider: SubagentProvider): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:136`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:138`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-removed--emit"></a>
 
@@ -675,7 +675,7 @@ A provider left the registry. Accepted runs remain holder-owned.
 'subagent/provider-removed'(name: string): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:142`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:144`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentstart--emit"></a>
 
@@ -699,5 +699,5 @@ A provider established a published child. For in-process providers, `ctx.agents.
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:153`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:155`](../../packages/subagent/subagent/src/index.ts)
 <!-- END GENERATED cordis-surface -->