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

examples/acp-agent/tests/acp.snapshot.ts

@@ -47,6 +47,9 @@ const SESSION_TITLE_CONFIG = fileURLToPath(new URL('../session-title.cordis.yml'
 const SUBAGENT_DURABILITY_FAILURE_CONFIG = fileURLToPath(
   new URL('../subagent-durability-failure.cordis.yml', import.meta.url),
 )
+const SUBAGENT_CONTINUABLE_INHERITANCE_CONFIG = fileURLToPath(
+  new URL('../subagent-continuable-inheritance.cordis.yml', import.meta.url),
+)
 const LSP_CONFIG = fileURLToPath(new URL('./lsp.cordis.yml', import.meta.url))
 const WEB_CONFIG = fileURLToPath(new URL('../web.cordis.yml', import.meta.url))
 const FS_SEARCH_CONFIG = fileURLToPath(new URL('./fs-search.cordis.yml', import.meta.url))
@@ -315,6 +318,18 @@ const SCENARIOS: Scenario[] = [
     pinsChildToolSchemas: [1],
     configPath: SUBAGENT_DURABILITY_FAILURE_CONFIG,
   },
+  // Authored policy-inheritance transcript: the root session is switched to
+  // read-only at creation (the UI Access switch equivalent), and the
+  // continuable background child's log carries that override as a
+  // `sandbox/mode` `source: 'delegation'` event, so the child's runtime
+  // context states the inherited policy instead of the deployment default.
+  {
+    name: 'subagent-continuable-inheritance',
+    hasModelTurn: true,
+    recorded: false,
+    pinsChildToolSchemas: [1],
+    configPath: SUBAGENT_CONTINUABLE_INHERITANCE_CONFIG,
+  },
   // The in-process child is published before its first follow-up fails. The
   // foreground tool retains both that run-result failure and an independent
   // published-handle disposal failure.