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/js/admin.js

@@ -912,6 +912,78 @@ function initEndpointForm() {
     btn.disabled = false; btn.textContent = 'Add';
   });
 
+  // GitHub Copilot — device-flow login. Starts the flow, shows the user a
+  // code + verification link, and polls until they authorise (or it expires).
+  const copilotBtn = el('adm-copilotConnectBtn');
+  if (copilotBtn) {
+    let copilotPolling = false;
+    copilotBtn.addEventListener('click', async () => {
+      if (copilotPolling) return;
+      const status = el('adm-copilotStatus');
+      const reset = () => { copilotBtn.disabled = false; copilotBtn.textContent = 'Connect GitHub Copilot'; copilotPolling = false; };
+      status.textContent = ''; status.className = 'adm-ep-inline-msg';
+      copilotBtn.disabled = true; copilotBtn.textContent = 'Starting...';
+      copilotPolling = true;
+      let start;
+      try {
+        const res = await fetch('/api/copilot/device/start', { method: 'POST', body: new FormData(), credentials: 'same-origin' });
+        start = await res.json();
+        if (!res.ok) { status.textContent = start.detail || 'Failed to start login'; status.className = 'admin-error'; reset(); return; }
+      } catch (e) { status.textContent = 'Request failed'; status.className = 'admin-error'; reset(); return; }
+
+      const { poll_id, user_code, verification_uri, verification_uri_complete, interval, expires_in } = start;
+      // Prefer the "complete" URL — it embeds the code so the user only has to
+      // click "Authorize" (no manual code entry).
+      const authUrl = verification_uri_complete || verification_uri || '';
+      const esc = (s) => String(s || '').replace(/[<>&"]/g, (c) => ({ '<': '&lt;', '>': '&gt;', '&': '&amp;', '"': '&quot;' }[c]));
+      copilotBtn.textContent = 'Waiting…';
+
+      // Cohesive waiting panel: spinner + status line, the device code as a
+      // copyable chip, and a primary "Authorize on GitHub" action.
+      status.className = '';
+      status.innerHTML =
+        '<div class="adm-copilot-panel">' +
+          '<div class="adm-copilot-wait"><span class="admin-spinner"></span>' +
+            '<span>Waiting for GitHub authorization…</span></div>' +
+          '<div class="adm-copilot-coderow">' +
+            '<span class="adm-copilot-code-label">Code</span>' +
+            '<code class="adm-copilot-code">' + esc(user_code) + '</code>' +
+            '<button type="button" class="admin-btn-sm adm-copilot-copy">Copy</button>' +
+          '</div>' +
+          '<a class="admin-btn-add adm-copilot-auth" href="' + encodeURI(authUrl) + '" target="_blank" rel="noopener">Authorize on GitHub ↗</a>' +
+          '<div class="adm-copilot-hint">A new tab opened on GitHub — approve there to finish. Didn\'t open? Use the button above.</div>' +
+        '</div>';
+      const copyBtn = status.querySelector('.adm-copilot-copy');
+      if (copyBtn) copyBtn.addEventListener('click', async () => {
+        try { await navigator.clipboard.writeText(user_code || ''); copyBtn.textContent = 'Copied'; setTimeout(() => { copyBtn.textContent = 'Copy'; }, 1500); } catch (e) {}
+      });
+      try { if (authUrl) window.open(authUrl, '_blank', 'noopener'); } catch (e) {}
+
+      const deadline = Date.now() + (expires_in || 900) * 1000;
+      const stepMs = Math.max((interval || 5), 2) * 1000;
+      const done = (cls, text) => { status.className = cls; status.textContent = text; reset(); };
+      const poll = async () => {
+        if (Date.now() > deadline) { done('admin-error', 'Authorization expired — try again.'); return; }
+        try {
+          const fd = new FormData(); fd.append('poll_id', poll_id);
+          const r = await fetch('/api/copilot/device/poll', { method: 'POST', body: fd, credentials: 'same-origin' });
+          const d = await r.json();
+          if (d.status === 'authorized') {
+            const n = ((d.endpoint && d.endpoint.models) || []).length;
+            done('admin-success', '✓ Connected — ' + n + ' Copilot model' + (n !== 1 ? 's' : '') + ' available.');
+            if (d.endpoint && d.endpoint.id) _recentlyAddedEpId = String(d.endpoint.id);
+            await loadEndpoints();
+            await _selectAddedModelInChat(d.endpoint || {});
+            return;
+          }
+          if (d.status === 'failed') { done('admin-error', 'Authorization failed (' + (d.error || 'denied') + ').'); return; }
+        } catch (e) { /* transient — keep polling */ }
+        setTimeout(poll, stepMs);
+      };
+      setTimeout(poll, stepMs);
+    });
+  }
+
   // Local "Add" button — sibling form for self-hosted base URLs.
   const localAddBtn = el('adm-epLocalAddBtn');
   const localTestBtn = el('adm-epLocalTestBtn');