docs(subagent): correct report acceptance semantics for closing parents

The tool README claimed a "missing, disposed, or closing parent" fails
the call — but acceptance is governed by the parent's registry presence:
`resolveReportParent` only rejects when the durable parent id is absent
from the registry, so a host-owned parent already in disposal but still
registered still accepts (the pinned host-disposing-parent behavior).
The claim misled callers into treating disposal state as a delivery
signal.

Restate the contract in both languages: absence from the registry is the
only `PARENT_UNAVAILABLE` case, and a failed tool call does not prove
non-delivery — a later `tools/post-execute` veto can fail a call whose
report was already accepted, so the durable child transcript remains the
recovery source.

Adds a regression test pinning acceptance into a host-disposing but
still-registered parent, and rejection after disposal settles.

packages/subagent/tool-subagent-report/tests/tool-subagent-report.spec.ts

@@ -350,6 +350,23 @@ describe('dsh-tool-subagent-report', () => {
     expect(ctx.agents.list().map(agent => agent.id)).toEqual([parent.id])
   })
 
+  it('accepts a report into a host-disposing but still-registered parent', async () => {
+    const { ctx } = await setup()
+    const parentHandle = await ctx.agents.create({
+      sessionId: SessionId('disposing-parent'),
+      agentOptions: { provider: 'mock', model: 'mock' },
+    })
+    const { child } = await startChild(ctx, parentHandle.agent)
+    // Host-owned disposal starts asynchronously; the parent stays registered
+    // until quiescence, and registry presence — not disposal state — is the
+    // acceptance gate (pins the README contract).
+    const disposing = parentHandle.dispose()
+    const accepted = await callReport(ctx, child, 'during-close')
+    expect(accepted.isError).toBe(false)
+    await disposing
+    expect((await callReport(ctx, child, 'after-close')).isError).toBe(true)
+  })
+
   it('keeps the namespace plugin shape and validates its default', () => {
     expect('default' in tool).toBe(false)
     expect(tool.name).toBe('tool-subagent-report')