Desktop: per-session mode, thinking/reasoning effort, and model selection — plus auto context-window from OpenAI-compatible /models metadata

### Summary

Desktop app: make chat mode, thinking/reasoning effort, and model selection per-session (not global/shared), and auto-derive each model's context window from the provider's OpenAI-compatible `/models` metadata instead of a hardcoded 128k fallback.

### Background / current behavior (Windows Desktop)

- Changing the context window or reasoning effort in one chat affects other chats — these are stored as shared provider/profile config, not per session.
- Third-party OpenAI-compatible models (my LiteLLM gateway) always show a `0/128k` context window, regardless of the model's real limit.
- There is no per-session way to set mode / reasoning / model from the UI — slash commands like `/model` and `/mode` do not work in the desktop session box.

### Requested behavior

1. **Per-session controls:** each chat has its own mode (Agent / Auto / Plan / Chat), thinking/reasoning effort, and model selection — independent of other chats, similar to the Claude Desktop app.
2. **Automatic context window:** detect the real context limit from the provider. My gateway already returns per-model metadata:

```bash
curl https://litellm.usc1.gcp.kibocommerce.com/v1/models \
-H "Authorization: Bearer $KEY"
```

Each model includes `max_input_tokens`, e.g. `1048576` for `glm-5.2`, `kimi-k3`, `deepseek-v4-pro`, and `262144` for `qwen3.8`. Goose should use that value for the `0/X` counter and for compaction, instead of defaulting to 128k.

3. Context window should be **per model per session**, so a mixed provider (some 1M, one 256k) needs no provider-duplication workaround.

### Environment

- OS & Arch: Windows
- Interface: Desktop (GUI)
- Provider: custom OpenAI-compatible (LiteLLM gateway)

### Related existing issues

- #11558 — live-first provider metadata incl. reasoning (recommend coordinating here)
- #11335 — per-chat backends / per-session scoping
- #11881 — closed: Windows GUI 128k default
- #11729 — stale desktop context counter

- [x] I checked for duplicates — this is the per-session scope + generic OpenAI-compatible `/models` auto-detect delta on top of #11558/#11335.