docs: add QVAC provider setup guide

[simon-iribarren]

What does this PR do?

Adds a provider setup page for QVAC and registers it in the providers index.

QVAC is an open-source runtime for local-first, peer-to-peer AI. It can expose your local models through an OpenAI-compatible HTTP server (qvac serve openai), so it connects to Roo Code through the existing OpenAI Compatible provider — no new provider code is required.

The page mirrors the format of the other local-provider pages (Ollama, LM Studio) and covers:

• Installing the QVAC CLI (npm i -g @qvac/cli).
• Defining a model alias in qvac.config.json and starting the server (default port 11434).
• Configuring the OpenAI Compatible provider in Roo Code: Base URL http://127.0.0.1:11434/v1, any non-empty API key, and a model id equal to the serve alias.

It also documents the non-obvious settings needed for QVAC to work well as a coding agent:

• Set ctx_size explicitly — the default of 1024 is too small for agent prompts.
• Set "tools": true so the model emits native tool calls instead of plain text (Roo Code uses native tool calling exclusively).
• For reasoning-tuned models such as Qwen3, set "reasoning_budget": 0.
• Tool-calling quality is bounded by the model, so a larger agent-tuned model such as gpt-oss-20b is recommended for local use.

Files changed

• docs/providers/qvac.md — new provider page.
• docs/providers/index.json — registers QVAC so it appears in the sidebar and the provider comparison table.

How was it tested?

Validated index.json, and the new page follows the same structure and frontmatter as the existing local-provider pages. The page uses no images and only absolute, extensionless internal links per the repo's documentation rules.

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