feat(chat): MCP server + WebSocket streaming, replace XML actions

- Backend: kanban binary gana subcomando `kanban mcp` que actua como MCP
  server via stdio. Tools = mismo set que executeTool (14). El subprocess
  llama de vuelta al backend via /api/tool/{name} con token interno.
- Backend: nuevo endpoint POST /api/tool/{name} (auth: X-Internal-Token).
- Backend: chat.go refactor — POST /api/chat reemplazado por GET
  /api/chat/ws (WebSocket). Lanza claude -p con --output-format stream-json
  --verbose --mcp-config y reenvia eventos (delta/tool_use/tool_result/
  result/done/error) como mensajes JSON al cliente.
- Backend: usa funciones nuevas del registry claude_stream_go_core (spawn
  + parser NDJSON) y mcp_server_stdio_go_infra (JSON-RPC stdio).
- Frontend: streamChat sobre WebSocket. ChatPanel renderiza deltas en
  vivo, chips para tool_use, badges teal/red para tool_result.
- Borrado: extractActions, actionsBlockMarker, XML system prompt.
- Tests: 7 nuevos en backend (chat_ws_test.go + endpoint /api/tool).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/handlers.go

@@ -330,7 +330,7 @@ func handlePurgeCard(db *DB) http.HandlerFunc {
 	}
 }
 
-func apiRoutes(db *DB, chatWorkdir string, logger *ChatLogger) []infra.Route {
+func apiRoutes(db *DB, chatWorkdir string, logger *ChatLogger, internalToken string) []infra.Route {
 	return []infra.Route{
 		{Method: "POST", Path: "/api/auth/register", Handler: handleRegister(db)},
 		{Method: "POST", Path: "/api/auth/login", Handler: handleLogin(db)},
@@ -353,6 +353,8 @@ func apiRoutes(db *DB, chatWorkdir string, logger *ChatLogger) []infra.Route {
 		{Method: "POST", Path: "/api/cards/{id}/restore", Handler: handleRestoreCard(db)},
 		{Method: "DELETE", Path: "/api/cards/{id}/purge", Handler: handlePurgeCard(db)},
 		{Method: "POST", Path: "/api/chat", Handler: handleChat(db, chatWorkdir, logger)},
+		{Method: "GET", Path: "/api/chat/ws", Handler: handleChatWS(db, chatWorkdir, logger, internalToken)},
+		{Method: "POST", Path: "/api/tool/{name}", Handler: handleInternalTool(db, internalToken, logger)},
 		{Method: "GET", Path: "/api/metrics", Handler: handleMetrics(db)},
 		{Method: "GET", Path: "/api/tags", Handler: handleListTags(db)},
 		{Method: "GET", Path: "/api/requesters", Handler: handleListRequesters(db)},