feat: implement web terminal for tool instances

- Add TerminalSession backend service for docker exec subprocess management
- Add TerminalManager for WebSocket session lifecycle management
- Create WebSocket endpoint at /ws/tool-instances/{id}/terminal
- Add session cookie authentication and instance ownership verification
- Install xterm.js with fit and web-links addons
- Create TerminalComponent with xterm.js integration
- Create TerminalPage with full-screen terminal view
- Add terminal route at /instances/:id/terminal
- Add terminal button to InstanceList for running instances
- Add terminal and arrow-left icons to icon registry
- Add comprehensive terminal CSS styles (dark theme, responsive)

Quality gates: typecheck ✓, lint ✓, build ✓, Python syntax ✓

apps/web/src/components/instance-list.tsx

@@ -1,4 +1,5 @@
 import { useCallback, useEffect, useState } from "react";
+import { useNavigate } from "react-router-dom";
 import { Icon } from "./icon";
 import type { ToolInstance } from "../api/sessions";
 import {
@@ -18,6 +19,7 @@ interface InstanceListProps {
 }
 
 export const InstanceList = ({ projectId, repoId, toolTypes }: InstanceListProps) => {
+  const navigate = useNavigate();
   const [instances, setInstances] = useState<ToolInstance[]>([]);
   const [loading, setLoading] = useState(false);
   const [showCreate, setShowCreate] = useState(false);
@@ -154,6 +156,16 @@ export const InstanceList = ({ projectId, repoId, toolTypes }: InstanceListProps
                     Open
                   </a>
                 )}
+                {instance.status === "running" && (
+                  <button
+                    className="secondary-button small"
+                    onClick={() => navigate(`/instances/${instance.id}/terminal`)}
+                    type="button"
+                  >
+                    <Icon name="terminal" size="sm" />
+                    Terminal
+                  </button>
+                )}
                 {instance.status !== "running" && (
                   <button
                     className="secondary-button small"