Add Agt — self-building agent framework with built-in OpenRouter provider

[vgp7758]

Adds apps/agt/ (app.yaml + 256x256 logo).

Agt is an agent framework that builds itself — the majority of its repo iterations are done by Agt running on its own code (dogfooding; a quine that evolves). Every module is hand-written with zero dependency on LangChain / LlamaIndex / AutoGen.

OpenRouter integration:

• Ships with OpenRouter as a built-in provider in its preset catalog (base_url https://openrouter.ai/api/v1, BYO-key onboarding flow straight from the model dropdown)
• 18 curated preset models incl. openrouter/auto, the :free pool, and flagships across Anthropic/OpenAI/Google/DeepSeek/Qwen/Z.ai/xAI/Moonshot
• Sends X-Title / HTTP-Referer attribution headers automatically when pointed at OpenRouter, so Agt users show up as Agt on the public rankings

It's a young project (traction is early — hence appealing to the uniqueness exception), but the self-building angle is genuine and everything above is verifiable in the repo.

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