feat: add terminal spinner for CLI agent mode

Fixes #576: shows a rotating Braille dot animation with 'Thinking...'
on stderr while waiting for LLM responses. Spinner runs in all three
agent modes (single message, interactive, simple interactive).

cmd/picoclaw/cmd_agent.go

@@ -65,6 +65,7 @@ func agentCmd() {
 	cfg, err := loadConfig()
 	if err != nil {
 		fmt.Printf("Error loading config: %v\n", err)
+		fmt.Println("Run 'picoclaw doctor' to check for common problems.")
 		os.Exit(1)
 	}
 
@@ -75,6 +76,7 @@ func agentCmd() {
 	provider, modelID, err := providers.CreateProvider(cfg)
 	if err != nil {
 		fmt.Printf("Error creating provider: %v\n", err)
+		fmt.Println("Run 'picoclaw doctor' to check for common problems.")
 		os.Exit(1)
 	}
 	// Use the resolved model ID from provider creation
@@ -105,7 +107,10 @@ func agentCmd() {
 
 	if message != "" {
 		ctx := context.Background()
+		spin := newSpinner("Thinking...")
+		spin.Start()
 		response, err := agentLoop.ProcessDirect(ctx, message, sessionKey)
+		spin.Stop()
 		if err != nil {
 			fmt.Printf("Error: %v\n", err)
 			os.Exit(1)
@@ -157,7 +162,10 @@ func interactiveMode(agentLoop *agent.AgentLoop, sessionKey string) {
 		}
 
 		ctx := context.Background()
+		spin := newSpinner("Thinking...")
+		spin.Start()
 		response, err := agentLoop.ProcessDirect(ctx, input, sessionKey)
+		spin.Stop()
 		if err != nil {
 			fmt.Printf("Error: %v\n", err)
 			continue
@@ -192,7 +200,10 @@ func simpleInteractiveMode(agentLoop *agent.AgentLoop, sessionKey string) {
 		}
 
 		ctx := context.Background()
+		spin := newSpinner("Thinking...")
+		spin.Start()
 		response, err := agentLoop.ProcessDirect(ctx, input, sessionKey)
+		spin.Stop()
 		if err != nil {
 			fmt.Printf("Error: %v\n", err)
 			continue

cmd/picoclaw/spinner.go

@@ -0,0 +1,62 @@
+// PicoClaw - Ultra-lightweight personal AI agent
+// License: MIT
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"sync"
+	"time"
+)
+
+// spinner displays a rotating progress indicator on stderr while the agent
+// processes a request. Output goes to stderr so that piped stdout is not
+// affected.
+type spinner struct {
+	frames  []rune
+	text    string
+	stop    chan struct{}
+	stopped chan struct{}
+	once    sync.Once
+}
+
+func newSpinner(text string) *spinner {
+	return &spinner{
+		frames:  []rune{'⠋', '⠙', '⠹', '⠸', '⠼', '⠴', '⠦', '⠧', '⠇', '⠏'},
+		text:    text,
+		stop:    make(chan struct{}),
+		stopped: make(chan struct{}),
+	}
+}
+
+// Start launches the spinner animation in a background goroutine.
+func (s *spinner) Start() {
+	go func() {
+		defer close(s.stopped)
+		i := 0
+		ticker := time.NewTicker(100 * time.Millisecond)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-s.stop:
+				// Clear the spinner line
+				fmt.Fprintf(os.Stderr, "\r\033[K")
+				return
+			case <-ticker.C:
+				frame := s.frames[i%len(s.frames)]
+				fmt.Fprintf(os.Stderr, "\r%c %s", frame, s.text)
+				i++
+			}
+		}
+	}()
+}
+
+// Stop signals the spinner to halt and waits for it to finish.
+func (s *spinner) Stop() {
+	s.once.Do(func() {
+		close(s.stop)
+		<-s.stopped
+	})
+}