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/snapshots/subagent-continuable-inheritance/input.json

@@ -0,0 +1,19 @@
+{
+ "steps": [
+  {
+   "op": "initialize"
+  },
+  {
+   "op": "newSession"
+  },
+  {
+   "op": "prompt",
+   "text": "Follow these steps exactly, then stop. 1. Call the subagent tool once with run_in_background set to true, description 'Reply with CHILD_OK', and prompt 'Reply with exactly the word CHILD_OK and nothing else.'. 2. Reply with the single word DONE. Do not use the bash tool."
+  },
+  {
+   "op": "waitForSubagentTurnEnd",
+   "child": 1,
+   "minimumTurn": 1
+  }
+ ]
+}