Feature request: support fresh external follow-up and remote chat into the active ACP session

## Feature request

Support a true external follow-up / remote chat surface for an active ACP session (for example via Discord), instead of only relying on stale status summaries or one-way notifications.

## Problem

When using VS Code + ACP as the main workspace, I may step away from VS Code and follow up from another surface such as Discord.

Right now, the external follow-up experience tends to feel stale because it is based on snapshots/notifications rather than a live connection to the active ACP session.

Two concrete needs:

1. **Fresh updates**
- external status requests should reflect the current state of the active ACP session, not an old snapshot
2. **True remote chat into the same session**
- I want to be able to message the active Lead/agent from Discord and continue the same VS Code session remotely

## Desired behavior

### A. Fresh status for active ACP session
If I request an update from an external surface, I want the response to be based on the **currently active ACP session**:
- current phase / current turn state
- last activity time
- whether the agent is still actively working
- whether the session is waiting for input / blocked / done

### B. Remote continuation of the same ACP session
If I reply from an external surface (for example Discord), I want that message to be able to:
- target the currently active ACP session
- continue the same conversation
- deliver the agent's reply back to the external surface
- effectively behave like a remote chat client for the same VS Code ACP session

## Why this matters

For multi-step coding workflows, the main VS Code chat is often the canonical session, but the user may need to:
- ask for progress remotely
- nudge the Lead to continue
- answer a question without returning to VS Code immediately

Without a real bridge, external follow-up becomes:
- stale
- one-way
- not trustworthy for active workflow management

## Possible implementation directions

Examples of approaches that could help:
- expose the active ACP session as a resumable remote endpoint
- add an extension-managed “active session bridge” abstraction
- distinguish between:
- notification-only integrations
- true bidirectional remote chat integrations
- provide an API/event stream for current active-session state

## Related issue

This is related to early quieting / incomplete in-progress behavior, but is broader than that issue:
- #19

That issue is about sessions appearing to go quiet too early.
This request is about making external follow-up into the active ACP session a first-class capability.

Start by reviewing the existing external status or notification flow and the active ACP session handling; the issue names no files, tests, or entry points. Use related issue #19 for context, then define the session bridge and bidirectional behavior needed for fresh status, remote messages, and replies delivered back to the external surface.

Written by the indexing model from the issue text.