docs(plugins): an agent.open.before hook starts writes to its own agent without awaiting

An agent.open.before hook that awaits setMetadata (or any write) on the
agent being opened deadlocks: the write joins the agent's lifecycle lane
behind the open, and the open waits on the hook until it times out. The
hooks reference now says so and shows the supported shape, a write
started without awaiting, which runs right after the open.

Deferred, not in this stack: reject such a write immediately instead of
letting the hook time out. The host knows which plugin hook it is
awaiting for the agent and which channel the write arrives on, so the
rejection can be keyed on "this plugin is inside an open hook for this
agent" without cross-process tagging, and an unrelated client's write
would still wait as it does today.

docs/plugins.md

@@ -474,6 +474,9 @@ path shape: a plugin can place a worktree several directories below the default
 Emit from the operation owner, not a client subscription. Provider history replay must not trigger
 live hooks. Notifications must not be awaited inside agent mutations: a handler can send a prompt
 or answer a permission through its own daemon session. Awaiting it there deadlocks that command.
+The same holds for a hook that runs inside an agent's lifecycle lane: an `agent.open.before` hook
+that awaits a write to the agent being opened (`setMetadata`, `setLabels`) waits on the open, which
+waits on the hook. The hooks reference tells plugin authors to start such a write without awaiting it.
 
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