Allow local ACP agents to opt in to follow-work-state / continuation mode

[cjaliaga]

Summary

Local ACP agents (ACP Registry entries launched with a local command) cannot surface permission requests, elicitations or updates that arrive while no session/prompt is active. The mechanism that handles this already exists in T3 ("follow agent work state" with continuation runs), but it is enabled only for specific built-in agents selected by agent id or command name, with no way for other agents to opt in.

Problem

Some agents keep working after a prompt turn has ended. A typical case: the agent started a background shell during the turn, the turn ended, and when the shell completes the agent resumes on its own to collect the result and continue (possibly running more tools).

With the generic registry flavor, anything the agent sends during that period is lost to the user:

• session/request_permission fails with ACP agent requested input without an active turn, regardless of the runtime/approval policy, because the active turn is resolved before the policy is read. Nothing is shown to the user; the agent only gets an error.
• Elicitations take the same path and fail the same way.
• session/update is only appended to the loaded history snapshot. It does not open a turn and the user is not notified.

As a result the agent's work stalls on a permission or question the user never sees, until the user happens to send another prompt in that thread.

What already exists

When follow-work-state mode is enabled (protocol version 2, state_update running/idle, continuation requests), a request or update that arrives while the thread is idle makes T3 offer a continuation run and attaches the request to it. A post-settle continuation wake also exists. Today these are enabled only for particular built-in agent flavors, keyed on agent id or command name.

Proposal

Let any ACP agent opt in, for example by either:

1. An agent capability advertised in the initialize response (e.g. a flag saying the agent reports work state and may continue after a turn ends); or
2. A per-agent setting for ACP Registry / local-command agents (e.g. "Follow agent work state" and "Allow continuation after settle").

With the opt-in, the existing follow-work-state behavior would apply: an idle permission request, elicitation or update starts (or offers) a continuation run and is shown to the user with the normal approval UI and policy.

Matching a built-in agent's id or command name to get this behavior is not an acceptable workaround for third-party agents.

Expected behavior

• An opted-in agent that sends session/request_permission or an elicitation while idle gets it shown to the user under the thread's normal policy, and receives the user's answer.
• An opted-in agent's session/update while idle is visible in the thread (as part of a continuation run), not only stored in history.
• Agents that do not opt in keep the current behavior.

Environment

• ACP agent added through the ACP Registry with a local command (generic registry flavor).