refactor: modularize TUI, improve error handling, add CI caching and tests

Split monolithic app.go into focused modules (dashboard, chat, workflow,
config, agents, terminal, commands, handlers). Add proper error handling
for installer commands, proxy pipes, and MCP config parsing. Fix daemon
channel buffer, cap orchestrator history, compile think regex once, and
set HTTP timeouts on preview server. Improve CI with Go module caching,
dependency download step, and test stage with race detection.

😘 Generated with Crush

Assisted-by: GLM-5-Turbo via Crush <crush@charm.land>

cmd/muyue/main.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
-	"strings"
 
 	tea "github.com/charmbracelet/bubbletea"
 	"github.com/muyue/muyue/internal/config"
@@ -518,22 +517,3 @@ func runSkills(args []string) {
 		fmt.Println("Available: list, show, generate, deploy, init, delete")
 	}
 }
-
-func checkConfigProviders(cfg *config.MuyueConfig) {
-	for i := range cfg.AI.Providers {
-		if cfg.AI.Providers[i].Active {
-			return
-		}
-	}
-	if len(cfg.AI.Providers) > 0 {
-		cfg.AI.Providers[0].Active = true
-	}
-}
-
-func joinWithQuotes(items []string) string {
-	quoted := make([]string, len(items))
-	for i, item := range items {
-		quoted[i] = fmt.Sprintf("%q", item)
-	}
-	return strings.Join(quoted, ", ")
-}