feat(webchat): add slash commands, autocomplete popup, and web search button

Add 6 slash commands (/help, /reset, /compact, /usage, /status, /model)
with autocomplete popup (arrow keys, Enter/Tab/Escape navigation).
Search button toggles web search mode by prepending instruction to message.
Backend agent.send extended with metadata for server-side command routing.

src/gateway/handlers/agent.ts

@@ -15,8 +15,8 @@ export interface AgentHandlerDeps {
 export function createAgentHandlers(deps: AgentHandlerDeps) {
   return {
     'agent.send': async (request: GatewayRequest, send: SendFn): Promise<OutboundMessage | void> => {
-      const params = request.params as { message?: string; connectionId?: string; attachments?: GatewayAttachment[] } | undefined;
-      if (!params?.message) {
+      const params = request.params as { message?: string; connectionId?: string; attachments?: GatewayAttachment[]; metadata?: { isCommand?: boolean; command?: string } } | undefined;
+      if (!params?.message && !params?.metadata?.isCommand) {
         return makeError(request.id, ErrorCode.InvalidRequest, 'message is required');
       }
 
@@ -43,6 +43,20 @@ export function createAgentHandlers(deps: AgentHandlerDeps) {
       return deps.laneQueue.enqueue(laneId, async () => {
         deps.sessionBridge.setBusy(connectionId, true);
 
+        // Handle slash commands via metadata (mirrors daemon/routing.ts pattern)
+        if (params.metadata?.isCommand) {
+          try {
+            if (params.metadata.command === 'reset') {
+              agent.reset();
+              send(makeEvent(request.id, 'done', { content: 'Session reset.' }));
+              return;
+            }
+          } finally {
+            deps.sessionBridge.setBusy(connectionId, false);
+            deps.metrics?.endRequest(requestId);
+          }
+        }
+
         // Set up tool use callback to emit streaming events
         deps.sessionBridge.setOnToolUse(connectionId, (event) => {
           if (event.type === 'start') {