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/tests/tool-subagent.spec.ts

@@ -510,4 +510,19 @@ describe('dsh-tool-subagent', () => {
     expect(seen?.toolFilter).toEqual({ deny: ['subagent'] })
     expect(seen?.toolFilter).not.toHaveProperty('allow')
   })
+
+  it('an explicit empty toolFilter fails at plugin load, not at first delegation', async () => {
+    const ctx = new Context()
+    await ctx.plugin(SystemPrompt)
+    await ctx.plugin(ToolRegistry)
+    await ctx.plugin(SubagentService)
+    ctx.subagents.registerProvider({
+      name: 'p',
+      capabilities: { outputSchema: false, depthLimit: false, toolFilter: true, persona: false },
+      inheritsParentContext: false,
+      start: () => { throw new Error('unreachable') },
+    })
+    const fiber = ctx.plugin(tool, { provider: 'p', toolFilter: {} })
+    await expect(fiber).rejects.toThrow(/names neither `allow` nor `deny`/)
+  })
 })