feat: add intercom extension API for typed subagent to root messaging

[avtc]

What

Adds a public extension API to pi-intercom that allows other extensions running in headless subagent sessions to communicate with the root session (which has UI access) through structured, typed messages.

Why

Previously, pi-intercom had hardcoded handlers for specific message types (permission requests, interview requests). Every new extension that needed to interact with the user through the root session required modifying pi-intercom itself.

This PR replaces that coupling with a generic, extensible protocol. Any extension can now register a handler and send typed messages to the root session — pi-intercom handles routing, serialization, and reply delivery without knowing anything about the specific use case.

How

API surface

The API is emitted via an intercom:ready event on pi.events (fired during session_start, after all extensions have loaded — no load-order issues):

Root session (hasUI=true) — register a handler:

pi.events.on("intercom:ready", (api) => {
  api.registerHandler("ask_user_question", async (ctx, from, message) => {
    // message.payload = { questions: Question[] }
    // ... present to user, collect answer ...
    return { text: JSON.stringify({ questions, answers, cancelled: false }) };
  });
});

Subagent session — send and wait for reply:

pi.events.on("intercom:ready", (api) => {
  const reply = await api.sendAndWait({
    contentType: "ask_user_question",
    payload: { questions: [...] },
    text: "Ask user question from subagent (2 question(s))",
    timeoutMs: Infinity,
  });
  // reply.text = JSON.stringify({ questions, answers, cancelled })
});

Key pieces

• Typed message routing — contentType + payload fields on Message; a registry (Map<string, Handler>) routes incoming messages to the correct handler
• sendAndWait() — queued, serialized RPC-style call from subagent to root session; concurrent calls chain sequentially so only one reply waiter is active at a time
• Configurable timeout — replyTimeoutMs config option (default 10 min, supports Infinity)
• Reload-safe listeners — globalThis-backed unsubscribe array prevents listener stacking on /reload
• Decoupled — removed all hardcoded handlers; pi-intercom is now a neutral transport layer

Tested with

pi-askuserquestion — PR (ghoseb/pi-askuserquestion#1)