feat: expose provider-neutral ACP goal extension (#964)

## Summary

- advertise a provider-neutral goal extension in
`InitializeResponse._meta.goal`
- map Claude SDK `active_goal` messages to
`session_info_update._meta.goal`
- expose the shared `_session/goal` control method with the truthfully
supported `clear` action
- document the extension as a candidate shape for a future first-class
ACP goal API

## Architecture

A goal is persistent **session state**, not the lifecycle of one
`session/prompt` request. `goal.status === "active"` says that the
objective may drive more work; it does not say that a prompt is
executing now.

This keeps two independent axes:

1. **Goal state** is published asynchronously as session metadata and
may survive multiple prompt turns.
2. **Prompt activity** follows the backend's actual running/quiescent
boundaries. While a turn runs, clients use advertised steering or prompt
queueing. Once the backend is quiescent, an ordinary `session/prompt` is
valid even if the goal remains active.

The metadata is deliberately provider-neutral:

```json
{
  "_meta": {
    "goal": {
      "version": 1,
      "controlMethod": "_session/goal",
      "actions": ["clear"]
    }
  }
}
```

State is emitted as `session_info_update._meta.goal`; `null` clears it.
The common snapshot vocabulary supports `objective`, common statuses,
timestamps in Unix milliseconds, budgets/usage, iteration count, and a
continuation reason. Each implementation emits only fields it actually
knows. This shape is intentionally close to a future typed ACP
capability/update so promotion out of `_meta` would not require a
provider-specific migration.

Control actions are capability-negotiated. The shared vocabulary
reserves `set`, `pause`, `resume`, and `clear`, but an agent advertises
only the subset it can implement with correct semantics.

## Claude mapping

The Claude runtime already emits internal `active_goal` messages,
although that message is not currently included in the public
`SDKMessage` union. The adapter handles it before the exhaustive switch:

- `condition` → `objective`
- `iterations` → `iterations`
- `set_at` → `createdAt`
- `last_reason` → `lastReason`
- `value: null` → `goal: null`

Provider bookkeeping such as `tokens_at_start` is intentionally not
exposed. Goal updates do not settle, extend, or otherwise mutate ACP
prompt lifecycle. Claude's existing Stop hook remains authoritative for
whether the backend itself continues working.

Only `clear` is advertised for out-of-band control. It submits `/goal
clear` through the existing session prompt queue, preserving the
adapter's established ordering and turn ownership. Setting a goal
remains available through `/goal <condition>` but is not advertised as a
control action because it starts the long-running Stop-hook workflow.

The companion Codex implementation is agentclientprotocol/codex-acp#371.

## Verification

- `npm run test:run` — 692 passed, 20 skipped
- `npm run build`
- `npm run check`

README.md

@@ -16,6 +16,7 @@ This tool implements an ACP agent by using the official [Claude Agent SDK](https
 - Interactive (and background) terminals
 - Custom [Slash commands](https://docs.anthropic.com/en/docs/claude-code/slash-commands)
 - Client MCP servers
+- Session-scoped long-running goals through the provider-neutral [goal extension](docs/goal-extension.md)
 
 Learn more about the [Agent Client Protocol](https://agentclientprotocol.com/).