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

static/index.html

@@ -2092,6 +2092,13 @@
                   <button class="admin-btn-add" id="adm-epAddBtn" style="width:55px;text-align:center;">Add</button>
                 </div>
                 <div id="adm-epApiMsg" class="adm-ep-inline-msg"></div>
+                <div class="adm-copilot-connect">
+                  <button class="admin-btn-sm" id="adm-copilotConnectBtn" type="button" title="Sign in to GitHub Copilot via device flow">
+                    <svg width="13" height="13" viewBox="0 0 24 24" fill="currentColor" style="vertical-align:-2px;margin-right:5px;opacity:0.8"><path d="M12 .5C5.7.5.5 5.7.5 12c0 5.1 3.3 9.4 7.9 10.9.6.1.8-.2.8-.5v-1.7c-3.2.7-3.9-1.5-3.9-1.5-.5-1.3-1.3-1.7-1.3-1.7-1.1-.7.1-.7.1-.7 1.2.1 1.8 1.2 1.8 1.2 1 1.8 2.7 1.3 3.4 1 .1-.8.4-1.3.7-1.6-2.6-.3-5.3-1.3-5.3-5.7 0-1.3.4-2.3 1.2-3.1-.1-.3-.5-1.5.1-3.1 0 0 1-.3 3.3 1.2a11.4 11.4 0 0 1 6 0C17.3 4.7 18.3 5 18.3 5c.6 1.6.2 2.8.1 3.1.8.8 1.2 1.8 1.2 3.1 0 4.4-2.7 5.4-5.3 5.7.4.4.8 1.1.8 2.2v3.3c0 .3.2.6.8.5 4.6-1.5 7.9-5.8 7.9-10.9C23.5 5.7 18.3.5 12 .5z"/></svg>
+                    Connect GitHub Copilot
+                  </button>
+                  <div id="adm-copilotStatus" class="adm-ep-inline-msg"></div>
+                </div>
               </div>
             </div>
           </div>