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

packages/subagent/subagent/src/index.ts

@@ -100,13 +100,15 @@ export { SubagentError } from './error.ts'
 export { settleRun } from './run-settlement.ts'
 export { assertSubagentMaxDepth, delegationDepthOf } from './depth.ts'
 export {
+  appendDelegatedPolicyOverrides,
   applyChildComposition,
+  captureDelegatedPolicyOverrides,
   childSessionMeta,
   resolveChildAgentOptions,
   resolveChildDepth,
   SubagentDepthError,
 } from './child-agent.ts'
-export type { ChildComposition } from './child-agent.ts'
+export type { ChildComposition, DelegatedPolicyOverrides } from './child-agent.ts'
 export type {
   ContinuableStart,
   ContinuableStartSpec,