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/icon.tsx

@@ -32,6 +32,8 @@ import {
   ArrowSquareOut,
   Play,
   Stop,
+  Terminal,
+  ArrowLeft,
 } from "@phosphor-icons/react";
 
 export type IconName =
@@ -71,7 +73,9 @@ export type IconName =
   | "binary"
   | "external"
   | "play"
-  | "stop";
+  | "stop"
+  | "terminal"
+  | "arrow-left";
 
 const iconMap: Record<IconName, React.ComponentType<{ size?: number | string; weight?: "thin" | "light" | "regular" | "bold" | "fill" | "duotone" }>> = {
   dashboard: House,
@@ -111,6 +115,8 @@ const iconMap: Record<IconName, React.ComponentType<{ size?: number | string; we
   external: ArrowSquareOut,
   play: Play,
   stop: Stop,
+  terminal: Terminal,
+  "arrow-left": ArrowLeft,
 };
 
 export interface IconProps {