fix: honor the teardown order on owner unload; make the structured commit unconditional

Adversarial-review findings (own reviewer agent), each verified and pinned:

B1: agents.register() returned a wrapper lambda, so the factory composite's
yield could not identity-nest it — on OWNER unload the unregistration (and
agent/disposed) disposed as a concurrent sibling, firing mid-drain while
the final turn was still closing (pre-existing on master; this branch's
docs re-assert the order, so it must be true). register() now returns the
EXACT cordis effect disposer (the Scope.rawDispose move); the composite
nests it and owner unload runs stop/drain -> unregister -> detach -> scope
like every other path. Regression test pins turn-end before disposed
before detach on owner unload.

B2: the structured two-phase commit could promote a stale stage when a
later capture call REUSED the orphaned stage's call id with a body that
never staged (denied downstream, or invalid args throwing pre-stage). The
runtime's pre-execute listener now clears any stale stage unconditionally
when a new capture call enters the pipeline — only a call's own body can
stage for its commit; the call-id mismatch guard becomes a defensive
second layer. Repro test: blocked capture then same-id invalid call.

C1: an explicit empty toolFilter config now fails at plugin LOAD (the
check is self-contained) instead of killing every delegation at child
setup. C2: Scope.dispose/ScopeHost.dispose @returns state the single-shot
repeat-call semantics honestly.

packages/subagent/tool-subagent/src/index.ts

@@ -184,6 +184,12 @@ export function providerWording(inherits: boolean): { description: string; promp
 }
 
 export function apply(ctx: Context, config: Config): void {
+  // Misconfiguration fails loud AT LOAD (the check is self-contained): an
+  // explicit `toolFilter: {}` would otherwise pass the capability gate and
+  // kill every delegation later, in the child-setup `restrict({})` throw.
+  if (config.toolFilter !== undefined && config.toolFilter.allow === undefined && config.toolFilter.deny === undefined) {
+    throw new Error('tool-subagent: `toolFilter` is configured but names neither `allow` nor `deny` — remove the key or fill the filter')
+  }
   // The tool MIRRORS its provider's lifecycle instead of assuming load order:
   // the cordis Loader starts sibling entries concurrently, so "backend listed
   // first in cordis.yml" does not guarantee "provider registered first", and