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

pnpm-lock.yaml

@@ -5887,6 +5887,12 @@ importers:
       '@deepseek-ai/dsh-llm':
         specifier: workspace:^
         version: link:../../llm/llm
+      '@deepseek-ai/dsh-sandbox':
+        specifier: workspace:^
+        version: link:../../sandbox/sandbox
+      '@deepseek-ai/dsh-sandbox-policy':
+        specifier: workspace:^
+        version: link:../../sandbox/sandbox-policy
       '@deepseek-ai/dsh-scope':
         specifier: workspace:^
         version: link:../../core/scope
@@ -5914,6 +5920,9 @@ importers:
       '@deepseek-ai/dsh-tools':
         specifier: workspace:^
         version: link:../../core/tools
+      '@deepseek-ai/dsh-user-approval':
+        specifier: workspace:^
+        version: link:../../interaction/user-approval
       cordis:
         specifier: ^4.0.0-rc.7
         version: link:../../../vendor/cordis