fix: add RPC callback support for sendUserMessage in RPC mode

[vers-landing-github-prod]

Problem

When punkin runs in --mode rpc (as a reef agent), the remind_me tool's reminders never fire. There are two compounding issues:

1. The pi process is ephemeral — reef kills it with SIGTERM on agent_end. Any in-process timers are destroyed.
2. Even if alive, sendUserMessage calls this.prompt() which starts a new agent turn internally, but the RPC caller isn't listening for unsolicited events.

The reminders extension in reef fires timers and calls pi.sendUserMessage(msg, { deliverAs: "followUp" }). In RPC mode, this call silently does nothing useful.

Fix

Add RPC callback support to sendUserMessage() in AgentSession.

When PUNKIN_RPC_CALLBACK_URL env var is set, instead of calling this.prompt() directly, POST the message to {PUNKIN_RPC_CALLBACK_URL}/tasks:

{
  "prompt": "<the reminder message>",
  "source": "reminder",
  "agentName": "<from VERS_AGENT_NAME or PUNKIN_AGENT_NAME env, optional>"
}

This lets the RPC caller (reef) receive the callback and schedule a new task turn — which is the correct mechanism in RPC mode.

Key properties

• Backward compatible: if PUNKIN_RPC_CALLBACK_URL is not set, sendUserMessage works exactly as before
• Fire-and-forget: the POST is non-blocking; errors are logged to stderr but don't throw
• Debuggable: logs a message to stdout when using the callback path
• No new dependencies: uses the built-in fetch API

Usage

The companion reef PR sets PUNKIN_RPC_CALLBACK_URL when launching punkin agents in RPC mode, pointing to reef's /tasks endpoint. Reef's task handler then creates a new agent turn with the reminder message as the prompt.

Files changed

• packages/coding-agent/src/core/agent-session.ts — sendUserMessage() method: check for PUNKIN_RPC_CALLBACK_URL and POST to it if set