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

knip.json

@@ -48,6 +48,7 @@
         "headless-agent/tests/fixtures/e2b/e2b/bin.ts",
         "acp-agent/tests/snapshots/lsp-definition/workspace/subject.ts",
         "acp-agent/tests/fixtures/child-question-tripwire.ts",
+        "acp-agent/tests/fixtures/parent-sandbox-override.ts",
         "acp-agent/tests/fixtures/partial-landlock-sandbox.ts",
         "acp-agent/tests/fixtures/subagent-durability-failure.ts",
         "acp-agent/tests/fixtures/subagent-settlement-marker.ts",