feat(provider): add GitHub Copilot provider with device-flow auth (#1480)

* feat(provider): add GitHub Copilot provider with device-flow auth

Adds GitHub Copilot as a model provider, so Copilot models (gpt-4o/4.1/5,
Claude, Gemini, …) work through the normal chat + agent loop, incl. native
tool calling and vision.

Auth is one-click via the GitHub OAuth device flow; the access token is stored
as the endpoint's (encrypted) api_key and sent directly as `Authorization:
Bearer` (no Copilot-token exchange, no refresh — matching how editors talk to
the Copilot API). Copilot is a normal ModelEndpoint detected by host; the only
provider-specific behaviour is a small set of required request headers,
injected centrally.

Sign-in is available from Settings → model endpoints ("Connect GitHub
Copilot") and from chat via `/setup copilot`.

- src/copilot.py (new), routes/copilot_routes.py (new): constants, header
  builders, device-flow start/poll, model discovery, owner-scoped endpoint
  provisioning.
- src/llm_core.py, src/endpoint_resolver.py: detect `copilot`, inject headers,
  per-request x-initiator/vision.
- src/agent_loop.py: allowlist api.githubcopilot.com for native tool schemas.
- src/model_context.py: known context windows for Copilot (no unauthenticated
  /models probe).
- static/, README, tests/test_copilot*.py.

* Tidy copilot_routes: clarify supports_tools, note _PENDING is per-process

README.md

@@ -11,7 +11,7 @@
 A self-hosted AI workspace -- meant to be the self-hosted version of the UI experience you get from ChatGPT and Claude. But with more jank and fun. Running on your own hardware, with your own data -- local-first, privacy-first, and no trojan.
 
 ## Features
-  - **Chat** -- chat with any local model or API; adding them is super simple.<br>　<sub>vLLM · llama.cpp · Ollama · OpenRouter · OpenAI</sub>
+  - **Chat** -- chat with any local model or API; adding them is super simple.<br>　<sub>vLLM · llama.cpp · Ollama · OpenRouter · OpenAI · GitHub Copilot</sub>
   - **Agent** -- hand it tools and let it run the whole task itself.<br>　<sub>built on [opencode](https://github.com/anomalyco/opencode) · MCP · web · files · shell · skills · memory</sub>
   - **Cookbook** -- Scans your hardware, recommends models, click to download and serve.. easy!<br>　<sub>built on [llmfit](https://github.com/AlexsJones/llmfit) · VRAM-aware · GGUF / FP8 / AWQ · fit scoring · vLLM / llama.cpp serving</sub>
   - **Deep Research** -- multi-step runs that gather, read, and synthesize sources into a nice visual report.<br>　<sub>adapted from [Tongyi DeepResearch](https://github.com/Alibaba-NLP/DeepResearch)</sub>