docs: Add documentation for spice chat CLI command (#962)

* docs: Add documentation for `spice chat` CLI command

* fix link

* fix

* add more context in flags and examples

* fix

* fix

* fix

* Update website/docs/cli/reference/chat.md

Co-authored-by: Luke Kim <80174+lukekim@users.noreply.github.com>

* add description

* Update header

* Update website/docs/cli/reference/chat.md

Co-authored-by: Evgenii Khramkov <evgenii@spice.ai>

---------

Co-authored-by: Luke Kim <80174+lukekim@users.noreply.github.com>
Co-authored-by: Evgenii Khramkov <evgenii@spice.ai>

Author: 3 people

website/docs/cli/reference/chat.md

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+---
+title: 'chat'
+sidebar_label: 'chat'
+pagination_prev: null
+pagination_next: null
+description: 'spice chat CLI documentation'
+---
+
+Start an interactive or one-shot chat with a [model](/docs/components/models/index.md) registered in the Spice runtime.
+
+## Requirements
+
+- Spice runtime must be running
+- At least one model defined in `spicepod.yaml` and the model is ready
+
+## Usage
+
+### Interative Chat: Invoke the command without arguments to open a REPL
+
+```shell
+spice chat [flags]
+```
+
+### One-shot Chat: Pass a single message as the argument to send a one-shot chat request and print the response
+
+```shell
+spice chat [flags] [<message>]
+```
+
+## Flags
+
+- `--cloud` Send requests to a Spice Cloud instance instead of the local instance. Default: `false`.
+- `--http-endpoint <string>` Runtime HTTP endpoint. Default: `http://localhost:8090`.
+- `--model <string>` Target model for the chat request. When omitted, the CLI uses the single ready model or prompts for a choice if several models are ready.
+- `--temperature <float32>` Model temperature used for chat request. Default: `1`.
+- `--user-agent <string>` Custom `User-Agent` header sent with every request.
+
+## Examples
+
+When exactly one model is **ready**, `spice chat` opens a REPL that uses that model automatically:
+
+```shell
+> spice chat
+Using model: openai
+chat> hello
+Hello! How can I assist you today?
+
+Time: 0.57s (first token 0.53s). Tokens: 18. Prompt: 8. Completion: 10 (325.04/s).
+```
+
+When multiple models are **ready**, the command prompts for a selection before starting the REPL:
+
+```shell
+> spice chat
+Use the arrow keys to navigate: ↓ ↑ → ←
+? Select model:
+  ▸ openai
+    llama
+Using model: openai
+chat> hello
+Hello! How can I assist you today?
+
+Time: 0.55s (first token 0.43s). Tokens: 18. Prompt: 8. Completion: 10 (80.09/s).
+```
+
+Passing `--model` skips the prompt and directs the request to the specified model. The flag works both in REPL mode and in one‑shot mode:
+
+```shell
+# REPL
+spice chat --model openai
+chat> hello
+Hello! How can I assist you today?
+
+Time: 0.61s (first token 0.58s). Tokens: 18. Prompt: 8. Completion: 10 (285.90/s).
+```
+
+Single prompt:
+
+```shell
+# One‑shot
+spice chat --model openai "hello"
+Hello! How can I assist you today?
+
+Time: 1.10s (first token 0.80s). Tokens: 18. Prompt: 8. Completion: 10 (33.74/s).
+```