docs(providers): Together/Groq/Perplexity cookbook via custom_providers

Three worked recipes for OpenAI-compatible cloud providers, plus the
Copilot HTTP 401 auto-recovery info block and the GMI Cloud row in the
compatible providers table. All three additions were on the original
docs/custom-providers-cookbook branch but its merge base predated 1186
main commits, making the rebase impractical (84k+ line conflict).

Replays just the providers.md additions onto current main.

website/docs/integrations/providers.md

@@ -1190,6 +1190,113 @@ You can also select named custom providers from the interactive `hermes model` m
 
 ---
 
+### Cookbook: Together AI, Groq, Perplexity
+
+The cloud providers listed in [Other Compatible Providers](#other-compatible-providers) all speak OpenAI's REST dialect, so they wire up the same way under `custom_providers:`. Three worked recipes follow. Each drops into `~/.hermes/config.yaml` and the matching API key goes in `~/.hermes/.env`.
+
+#### Together AI
+
+Hosts open-weight models (Llama, MiniMax, Gemma, DeepSeek, Qwen) at prices significantly below first-party APIs. Good default for multi-model fleets.
+
+```yaml
+# ~/.hermes/config.yaml
+custom_providers:
+  - name: together
+    base_url: https://api.together.xyz/v1
+    key_env: TOGETHER_API_KEY
+    # api_mode: chat_completions  # default — no need to set
+
+model:
+  default: MiniMaxAI/MiniMax-M2.7   # or any model from together.ai/models
+  provider: custom:together
+```
+
+```bash
+# ~/.hermes/.env
+TOGETHER_API_KEY=your-together-key
+```
+
+Switch models mid-session:
+
+```
+/model custom:together:meta-llama/Llama-3.3-70B-Instruct-Turbo
+/model custom:together:google/gemma-4-31b-it
+/model custom:together:deepseek-ai/DeepSeek-V3
+```
+
+Together's `/v1/models` endpoint works, so `hermes model` can auto-discover available models.
+
+#### Groq
+
+Ultra-fast inference (~500 tok/s on Llama-3.3-70B). Small catalog but strong for latency-sensitive interactive use.
+
+```yaml
+# ~/.hermes/config.yaml
+custom_providers:
+  - name: groq
+    base_url: https://api.groq.com/openai/v1
+    key_env: GROQ_API_KEY
+
+model:
+  default: llama-3.3-70b-versatile
+  provider: custom:groq
+```
+
+```bash
+# ~/.hermes/.env
+GROQ_API_KEY=your-groq-key
+```
+
+#### Perplexity
+
+Useful when you want a model that does live web search and citation automatically. Strict about which models are available — check [perplexity.ai/settings/api](https://www.perplexity.ai/settings/api) for the current list.
+
+```yaml
+# ~/.hermes/config.yaml
+custom_providers:
+  - name: perplexity
+    base_url: https://api.perplexity.ai
+    key_env: PERPLEXITY_API_KEY
+
+model:
+  default: sonar
+  provider: custom:perplexity
+```
+
+```bash
+# ~/.hermes/.env
+PERPLEXITY_API_KEY=your-perplexity-key
+```
+
+#### Multiple providers in one config
+
+The three recipes compose — use all of them together and switch per turn with `/model custom:<name>:<model>`:
+
+```yaml
+custom_providers:
+  - name: together
+    base_url: https://api.together.xyz/v1
+    key_env: TOGETHER_API_KEY
+  - name: groq
+    base_url: https://api.groq.com/openai/v1
+    key_env: GROQ_API_KEY
+  - name: perplexity
+    base_url: https://api.perplexity.ai
+    key_env: PERPLEXITY_API_KEY
+
+model:
+  default: MiniMaxAI/MiniMax-M2.7
+  provider: custom:together      # boot to Together; switch freely after
+```
+
+:::tip Troubleshooting
+- `hermes doctor` should print no `Unknown provider` warnings for any of these names after the CLI validator fixes in #15083.
+- If a provider's `/v1/models` endpoint is unreachable (Perplexity is the common one), `hermes model` will persist the model with a warning rather than hard-reject — see #15136.
+- To skip `custom_providers:` entirely and use bare `provider: custom` with `CUSTOM_BASE_URL` env var, see #15103.
+:::
+
+---
+
 ### Choosing the Right Setup
 
 | Use Case | Recommended |