Build 11.5 task — Implement real LLM response path

[grumpystrongman]

Parent build: #2

Goal

Connect the existing Ask flow to a real provider response path so Threadline can answer using resolved context.

Work items

• Identify the current prompt composition path used by Ask.
• Route the composed prompt/context into the configured provider abstraction.
• Return assistant text to the Windows sidecar.
• Append the assistant response into the transcript.
• Add user-readable handling for missing provider configuration, provider timeout, and provider error responses.

Acceptance criteria

• Ask no longer stops at prompt composition.
• A configured provider can return an answer into the transcript.
• Provider failure produces a clean assistant/system message instead of a crash.
• Follow/Lock context behavior is respected when composing the provider request.

[grumpystrongman]

Retry status for this task:

The Windows side of the real LLM response path is implemented:

• ThreadlineLocalClient.AskAsync(...) calls POST /sessions/{sessionId}/ask.
• MainWindow.AskAsync() routes user questions into that method and appends the returned answer into the transcript.
• Missing /ask is handled with a typed exception and prompt-composition fallback so the sidecar remains usable with older service builds.

This task should remain open until the backend service route is implemented and verified. Required backend endpoint:

POST /sessions/{sessionId}/ask

Expected behavior:

1. Compose messages using the same context path as /prompt.
2. Send those messages to the active provider abstraction.
3. Return { answer, messages } to the Windows sidecar.
4. Return useful non-success responses for missing provider configuration, provider timeout, and provider failure.