subagent: seed inherited policy events at creation

The parent implementation introduced sandboxMode and approvalPolicy as generic SessionHeader fields, then propagated those fields through both persistence backends, session-query indexes, collision checks, policy-specific seed-boundary folds, catalogs, and a broad test matrix. That storage plane is unnecessary: Session already accepts a validated constructor seed, and persistence captures that seed when the session is announced before committing its first batch.

Capture each parent override synchronously at delegation, append source-tagged sandbox/mode and approval/policy records after the optional fork prefix, and create the child with that combined seed. Keeping header.seedLength at the original fork-prefix length preserves lineage while ordinary last-event-wins folds make the inherited records outrank stale parent history and remain subordinate to later child switches. Unswitched parents still stamp nothing, so children continue to follow deployment defaults.

Remove the generic header fields and every persistence/query/schema branch built around them. Collapse the inheritance suite from ten leaking scenarios to four owned-context cases covering real filesystem confinement, stale fork precedence, delegation-time capture, and the no-override path. The assembled headless snapshot now asserts the persisted inheritance event directly.

This keeps the security behavior while restoring policy ownership to the existing event log and deleting the speculative durability machinery that the original tests did not exercise.

docs/core-data-structures/persistence.md

@@ -72,30 +72,12 @@ interface SessionHeader {
    * resume — a runtime-only depth would reset a resumed child to top-level.
    */
   readonly delegationDepth?: number
-  /**
-   * The sandbox-mode override inherited from the delegating parent at
-   * creation (the delegation-inheritance baseline). A neutral string here:
-   * the policy owner (`dsh-sandbox-policy`) validates it against its closed
-   * vocabulary on every read, this being a durable boundary. Absent for
-   * top-level sessions and for children of unswitched parents, which keep
-   * following the LIVE deployment default. Header-carried (the
-   * `delegationDepth` precedent) so the baseline is durable from the creation
-   * moment — no first-turn event survives every crash window, because an
-   * idle injection can persist a complete turn before any prompt turn opens.
-   */
-  readonly sandboxMode?: string
-  /**
-   * The approval-policy override inherited from the delegating parent at
-   * creation. Same contract as {@link SessionHeader.sandboxMode}; validated
-   * by `dsh-user-approval` on read.
-   */
-  readonly approvalPolicy?: string
 }
 ```
 
 ## `CreateSessionOptions` — seeding and metadata
 
-Creating a `Session` through the store takes a `seed` (replay/fork an existing event log) and `meta` (the storage-level fields the store folds into a `SessionHeader`). The store fills in `version`/`id` and defaults `createdAt`; the caller supplies the validated absolute `cwd`, the `parentSession` lineage, the `seedLength` seed boundary, the `delegationDepth`, the inherited `sandboxMode`/`approvalPolicy` delegation baselines, and — only when reconstructing a persisted session — the original `createdAt` to preserve it.
+Creating a `Session` through the store takes a `seed` (replay/fork an existing event log) and `meta` (the storage-level fields the store folds into a `SessionHeader`). The store fills in `version`/`id` and defaults `createdAt`; the caller supplies the validated absolute `cwd`, the `parentSession` lineage, the `seedLength` seed boundary, the `delegationDepth`, and — only when reconstructing a persisted session — the original `createdAt` to preserve it.
 
 ```ts type-equiv
 /**
@@ -116,8 +98,6 @@ interface CreateSessionOptions {
     readonly createdAt?: number
     readonly seedLength?: number
     readonly delegationDepth?: number
-    readonly sandboxMode?: string
-    readonly approvalPolicy?: string
   }
 }
 ```