feat: add web search, file edit, MCP management, file refs, and /stop to CLI/TUI

1. Web search tools (multi-provider): web_search, web_search_news, web_search_providers
   - Brave, Tavily, SearXNG (env-var activated), DuckDuckGo (always available)
   - Auto mode tries providers in priority order

2. File edit tool (fs_edit_file): targeted text replacement with validation
   - Unique oldText matching, overlap detection, atomic application
   - Sandboxed with size limits

3. MCP management commands: /mcp status, /mcp servers, /mcp reconnect <name>
   - Shows connection status, tool counts, errors for all MCP servers
   - Reconnect disconnected servers from the TUI

4. File reference in chat: @path/to/file inline expansion + /attach command
   - Auto-detects @file patterns and inlines contents as fenced code blocks
   - 256KB per file limit, max 10 files per message
   - Language-aware syntax highlighting hints

5. /stop command: abort streaming via Pi SDK abort()
   - New AbortPayload type in @mosaic/types
   - Gateway abort handler calls piSession.abort()
   - TUI emits abort event on /stop

packages/types/src/chat/events.ts

@@ -99,6 +99,11 @@ export interface SetThinkingPayload {
   level: string;
 }
 
+/** Client request to abort the current agent operation */
+export interface AbortPayload {
+  conversationId: string;
+}
+
 /** Socket.IO typed event map: server → client */
 export interface ServerToClientEvents {
   'message:ack': (payload: MessageAckPayload) => void;
@@ -120,4 +125,5 @@ export interface ClientToServerEvents {
   message: (data: ChatMessagePayload) => void;
   'set:thinking': (data: SetThinkingPayload) => void;
   'command:execute': (data: SlashCommandPayload) => void;
+  abort: (data: AbortPayload) => void;
 }

packages/types/src/chat/index.ts

@@ -11,6 +11,7 @@ export type {
   SessionInfoPayload,
   RoutingDecisionInfo,
   SetThinkingPayload,
+  AbortPayload,
   ErrorPayload,
   ChatMessagePayload,
   ServerToClientEvents,