Expose plugin-initiated detached sessions on ExtensionActions (it already works via DaemonClient)

[jasoncarreira]

Summary

An extension can already start a detached session and prompt it without blocking its own turn — by importing
DaemonClient from the package and speaking the daemon protocol. It just is not part of the extension API or the
documentation, so it reads as unsupported. I would like it exposed on ExtensionActions (or documented as a
supported path), because it is the one capability our opencode plugin has that our prime extension cannot offer.

Environment. prime-agent 0.7.2, daemon protocol 7 (protocol-7-schema-16-1bcb9e7f1a49), build 83a0f9f-dirty, Node 24.11.1, macOS 15 (darwin 25.5.0).

What works today

import { DaemonClient, defaultDaemonSocketPath } from "prime-agent";

const client = new DaemonClient(defaultDaemonSocketPath());
await client.connect(5000);
await client.waitForHello(5000);

const created = await client.request({ type: "create", name: "my-background-run" });
const activeSessionId = created.data.activeSessionId;          // -> "d7190de5f08a"

await client.request({ type: "prompt", activeSessionId, message: "…" });   // fire and forget

Result: prompt returns success: true immediately, the session runs on its own, prime-agent list shows it
(lifecycle: live), and prime-agent stop d7190de5f08a returns ok. Everything a caller needs is already there.

What is missing

ExtensionActions has 14 members — sendMessage, sendUserMessage, appendEntry, setSessionName,
getSessionName, setLabel, getActiveTools, getAllTools, setActiveTools, refreshTools, getCommands,
setModel, getThinkingLevel, setThinkingLevel — and every one acts on the extension's own current session.
There is no way to start another one, so an extension that wants to dispatch background work has to reach around
its own API into DaemonClient, with no guarantee that remains supported.

A small addition would cover it, for example:

interface ExtensionActions {
  // …existing members…
  /** Start a detached session and return a handle usable with stop/attach/send. */
  createDetachedSession(options?: { name?: string; cwd?: string }): Promise<{ activeSessionId: string }>;
  promptSession(activeSessionId: string, message: string): Promise<void>;
}

Why it matters to us

We ship the same build-orchestration tool as an opencode plugin and as a prime extension. The opencode plugin
offers a background mode because its plugin API exposes session creation and an async prompt; our prime
extension is foreground-only for exactly the missing piece above. Making it a supported extension capability
would let the two adapters offer the same surface.

Not verified

I confirmed the sequence from an ordinary Node import against a live daemon, not from inside a loaded extension —
so I have shown the daemon accepts it, not that the extension runtime permits it. If that distinction matters,
I can test it inside a real extension.