Add "Ollama Cloud" as a provider option

**Motivation**

Buzz's agent provider picker has named entries for Anthropic, OpenAI, OpenRouter, and Databricks, but nothing for [Ollama Cloud](https://docs.ollama.com/cloud). Users who want to run hosted open models (e.g. `kimi-k3`, `glm-5.2`, `deepseek-v4-pro`) can technically get there today by picking "OpenAI-compatible" and manually typing the base URL, API key, and model id — but nothing in the UI hints this is possible, so most don't realise it's an option.

**Proposed solution**

Add "Ollama Cloud" as a named provider in the picker. A user selects it, pastes their Ollama API key into the credential field, and chooses a model from the auto-populated list — the same one-click flow the other named providers already offer. No manual env-var editing.

**Alternatives considered**

- Keep using the existing "OpenAI-compatible" entry (works today but undiscoverable — users have to know the magic base URL and that it's even an option).
- Fold this into #3410 (share-compute routing to external endpoints) — different concern: that centralizes a credential on a community serving node; this is per-user provider selection in the desktop picker.

**Additional context**

Ollama Cloud exposes an OpenAI-compatible endpoint, so no new wire format is needed — this is a discoverability and UX gap, not a capability gap.

Related: #3145 (local Ollama, sibling), #3410 (share-compute external endpoints).