subagent: carry inherited policy overrides in the child session header

Review fix (ds-review-bot critical #2 on #623): the first-turn event stamp
had a durability hole no turn anchoring can close — an idle SessionStart-
style injection persists a complete one-shot turn before any prompt turn
opens, so a crash in that window left a resumable-looking child with no
inherited policy, falling back to a possibly wider deployment default.

The captured overrides now ride the child's creation meta into its
immutable SessionHeader (sandboxMode/approvalPolicy, neutral strings at the
session boundary — the delegationDepth precedent), durable from the moment
the session exists: no listener ordering can starve the baseline and no
crash window can lose it. overrideOf(session) on both policy services
resolves fold(events past header.seedLength) ?? header baseline, validating
against the closed vocabulary on read; stampOverride and the prompt-submit
listener machinery are deleted. The header field rides both persistence
backends (JSONL header line; SQLite sessions columns, SCHEMA_VERSION 11 —
pre-release, no migration). pty-local reads through overrideOf so PTY
spawns see the baseline too.

Red-first: header-durability-before-any-turn test (the injection crash
window shape), baseline/seed-boundary/closed-vocabulary contract tests in
both service suites; the real-wall suite (race, veto, fork stale-seed,
grandchild) re-anchored on header assertions and green. The Agent Note's
Alternatives now records the superseded event-stamping iteration with the
review evidence; bilingual docs updated.

packages/subagent/subagent-inprocess/README.md

@@ -18,7 +18,7 @@ The driver follows this sequence:
 
 The child gets the parent's working-directory/session lineage and inherits the parent model unless `request.agentOptions` overrides it. It gets a fresh flat registration scope: parent ownership does not import parent tool restrictions or establish an authority subset.
 
-The child also inherits the parent's session POLICY overrides. The driver captures `ctx.sandboxPolicy.overrideOf(parent.session)` and `ctx.approval.overrideOf(parent.session)` synchronously before its first await — the delegation moment is the snapshot point, so a parent switch racing the asynchronous child creation belongs to the parent's future — and a one-shot PREPENDED `agent/prompt-submit` listener stamps the captured values through `stampOverride` (both services consumed opportunistically — compositions without them delegate policy-free). Anchoring inside the child's first turn keeps the stamp turn-enclosed (durable) and ahead of the first request; prepending puts it before veto-capable listeners, so a denying UserPromptSubmit hook cannot close the first turn without the stamp; its log position after any fork-seed switch lets the ordinary last-event-wins fold resolve stale-seed timing. Only the override chain is copied, so an unswitched parent stamps nothing and the child follows the live deployment default. Nesting composes: each capture folds the delegating session's already-stamped log ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
+The child also inherits the parent's session POLICY overrides. The driver captures `ctx.sandboxPolicy.overrideOf(parent.session)` and `ctx.approval.overrideOf(parent.session)` synchronously before its first await — the delegation moment is the snapshot point, so a parent switch racing the asynchronous child creation belongs to the parent's future — and carries the captured values in the child's creation meta into its immutable `SessionHeader` (`sandboxMode`/`approvalPolicy`), durable from the moment the session exists: no listener ordering can starve the baseline and no crash window can lose it, including an idle SessionStart-style injection persisting a complete turn before any prompt turn opens. Both services are consumed opportunistically — compositions without them delegate policy-free. Only the override chain is copied, so an unswitched parent writes no baseline and the child follows the live deployment default; `overrideOf` folds only events past the seed boundary, so a fork seed's stale switch is subsumed by the baseline while the child's own later switches outrank it. Nesting composes: each capture resolves the delegating session's own chain ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
 
 ## Cancellation and ownership