Stable 1.137: new Agent Host sessions stay on Working because first turn is never dispatched

- VS Code Version: Stable 1.137.0 (`645f29cc3176500b4b5762ba887cf2a7f0ffdf2c`)
- OS Version: macOS 26.6.2 (25G83), arm64
- Feature: Agents Window, local Agent Host, new Copilot session
- Selected model: GPT-5.6 Sol
- Extensions disabled: Not tested. The failure is between the built-in Agents Window client and the editor-owned Agent Host.

## Summary

In Stable 1.137.0, every fresh local Agent Host session I create accepts the first prompt and shows **Working** indefinitely, but the first turn is never dispatched to Agent Host.

I reproduced this three times. A direct AHP client connected to the same editor-owned Agent Host process, using the same repository and model, completed a first turn successfully. This isolates the failure to the Stable Agents Window client path rather than the Agent Host, Copilot provider, authentication, model, quota, or repository.

## Steps to reproduce

1. Open the Agents Window in Stable VS Code 1.137.0.
2. Create a new local Copilot Agent Host session for an existing Git repository.
3. Submit a trivial first prompt such as `hi`.
4. Wait several minutes.

## Expected

The prompt reaches Agent Host and the assistant responds. If dispatch fails, the turn terminates with a visible error.

## Actual

The prompt appears in the UI and the session remains on **Working** indefinitely. Stop is shown, but Agent Host has no active or completed turn for the prompt.

This reproduced 3/3 times in one Stable process at approximately 13:19, 13:53, and 14:15 local time.

## Evidence

For each attempt, the Agent Host successfully created a provisional chat:

```text
[Copilot] Chat created; its backing stays deferred until the first send: copilotcli:/
```

The renderer then reached:

```text
[AgentHost] _invokeAgent called for resource: agent-host-copilotcli:/
```

Processing stopped there. No matching `dispatchAction`, `chat/turnStarted`, provider send, terminal action, or error appeared in the Agent Host log.

For the surviving attempt, I inspected both SQLite databases read-only:

- Agent Host session database: `turns=0`, `local_turns=0`
- Linked backing SDK session database: `turns=0`, `local_turns=0`
- Both databases passed `PRAGMA quick_check`

The UI's repository changes badge was pre-existing repository state; neither database recorded file edits or turn usage from the prompt.

## Direct AHP control

I then used a disposable token-safe client against the exact same editor-owned Unix-socket endpoint and process:

1. Negotiated AHP 0.9.0.
2. Called `resolveSessionConfig` and `createSession` for the same repository.
3. Subscribed to the server-generated default chat.
4. Dispatched `chat/turnStarted` using GPT-5.6 Sol.
5. Received `chat/turnComplete` with `AHP_DIRECT_OK`.
6. Verified no repository changes and archived the diagnostic session.

```text
advertised_protocol=0.9.0
negotiated_protocol=0.9.0
initial_lifecycle=creating
terminal_action=chat/turnComplete
response=AHP_DIRECT_OK
```

This proves the host and provider can materialize and complete a first turn while the Stable Agents Window cannot send it.

## Assessment

The first observable divergence is after the renderer logs `_invokeAgent called` and before any `chat/turnStarted` action reaches Agent Host. The UI optimistically enters the active state without a corresponding protocol turn and has no bounded failure path.

The existing historical-session hydration problem seen in the same profile is intentionally excluded from this report. This issue is reproducible with brand-new zero-turn sessions.

## Related issues

- #334217 reports the same visible symptom on 1.137.0 **Insiders**, but does not establish the Stable failure boundary or include an AHP control.
- #330531 also produced Working forever with no model turn, but its failing path omitted `createSession`; here `createSession` succeeds and the first-turn action is missing.
- #331118 begins after `chat/turnStarted` during first-turn configuration refresh; here no turn reaches Agent Host.
- #332831 documents deferred lifecycle `creating`; in this Stable reproduction, direct AHP dispatch successfully materializes that state while the Agents Window does not dispatch.