Add documentation for Claude Opus 4.5, Gemini 3 Pro, and GPT-5.1 models (#1450)

Add documentation for newly supported LLM models and create
`public/llms.txt` for LLM discovery.

## Changes

### `docs/cody/capabilities/supported-models.mdx`
- Added **GPT-5.1** (OpenAI) with vision support
- Added **Claude Opus 4.5** and **Claude Opus 4.5 w/ Thinking**
(Anthropic) with vision support
- Added **Gemini 3 Pro** (Google) with vision support
- Removed duplicate Gemini 2.0 Flash entry (pre-existing issue)

### `public/llms.txt`
- Created comprehensive LLM listing for Chat/Prompts, Autocomplete, and
Smart Apply categories
- Includes all supported models with vision capability annotations

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> add documentation for the following models in
docs/cody/capabilities/supported-models.mdx and public/llms.txt:
> - Opus 4-5
https://platform.claude.com/docs/en/about-claude/models/overview
> - Gemini 3 Pro
https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/3-pro
> - GPT 5.1 https://platform.openai.com/docs/models/gpt-5.1


</details>



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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: julialeex <[email protected]>
Co-authored-by: julialeex <[email protected]>

Author: Copilot

docs/cody/capabilities/supported-models.mdx

@@ -13,6 +13,7 @@ Cody supports a variety of cutting-edge large language models for use in chat an
 | **Provider** | **Model**                                                                                                                                     | **Status**        | **Vision Support** |
 | :----------- | :-------------------------------------------------------------------------------------------------------------------------------------------- | :---------------- | :----------------- |
 | OpenAI       | [GPT-5](https://platform.openai.com/docs/models/gpt-5)                                                                                        | ✅                | ✅                 |
+| OpenAI       | [GPT-5.1](https://platform.openai.com/docs/models/gpt-5.1)                                                                                    | ✅                | ✅                 |
 | OpenAI       | [GPT-5-mini](https://platform.openai.com/docs/models/gpt-5-mini)                                                                              | ✅                | ✅                 |
 | OpenAI       | [GPT-5-nano](https://platform.openai.com/docs/models/gpt-5-nano)                                                                              | ✅                | ✅                 |
 | OpenAI       | [GPT-4-Turbo](https://platform.openai.com/docs/models/gpt-4-and-gpt-4-turbo#:~:text=TRAINING%20DATA-,gpt%2D4%2D0125%2Dpreview,-New%20GPT%2D4) | ✅                | ❌                 |
@@ -34,11 +35,12 @@ Cody supports a variety of cutting-edge large language models for use in chat an
 | Anthropic    | [Claude Opus 4.1](https://docs.anthropic.com/en/docs/about-claude/models/overview)                                                            | ✅                | ✅                 |
 | Anthropic    | [Claude Opus 4](https://docs.anthropic.com/en/docs/about-claude/models/overview)                                                              | ✅                | ✅                 |
 | Anthropic    | [Claude Opus 4 w/ Thinking](https://docs.anthropic.com/en/docs/about-claude/models/overview)                                                  | ✅                | ✅                 |
+| Anthropic    | [Claude Opus 4.5](https://docs.anthropic.com/en/docs/about-claude/models/overview)                                                            | ✅                | ✅                 |
 | Google       | [Gemini 1.5 Pro](https://deepmind.google/technologies/gemini/pro/)                                                                            | ✅ (beta)         | ✅\*               |
 | Google       | [Gemini 2.0 Flash](https://deepmind.google/technologies/gemini/flash/)                                                                        | ✅                | ✅\*               |
-| Google       | [Gemini 2.0 Flash](https://deepmind.google/technologies/gemini/flash/)                                                                        | ✅                | ✅\*               |
 | Google       | [Gemini 2.5 Pro](https://cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/2-5-pro)                                                 | ✅                | ✅\*               |
 | Google       | [Gemini 2.5 Flash](https://cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/2-5-flash)                                             | ✅                | ✅\*               |
+| Google       | [Gemini 3 Pro](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/3-pro)                                                | ✅                | ✅\*               |
 
 <Callout type="note">
 	* While Gemini models support vision capabilities, Cody clients do not