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

src/model_context.py

@@ -282,6 +282,16 @@ def _query_context_length(endpoint_url: str, model: str) -> int:
         except Exception:
             pass
 
+    # GitHub Copilot's /models requires auth + X-GitHub-Api-Version headers that
+    # aren't available here; an unauthenticated probe just 400s. All Copilot
+    # picker models are major API models covered by the known-context table, so
+    # rely on that instead of a doomed network call.
+    from src.copilot import is_copilot_base
+    if is_copilot_base(endpoint_url):
+        if known:
+            logger.info(f"Using known context window for {model}: {known}")
+        return known or DEFAULT_CONTEXT
+
     models_url = endpoint_url.replace("/chat/completions", "/models")
     try:
         r = httpx.get(models_url, timeout=REQUEST_TIMEOUT)