feat: crush-inspired TUI improvements, activity bar polish, startup fresh session

TUI improvements:
- semantic icon constants (✓ ◆ ▣ ✕ ⌘ ◌ ◔ ◉)
- pill-style metadata bar with model/provider/session/context
- fuzzy command palette with deterministic subsequence scoring
- input history navigation (ctrl+p / ctrl+n)
- structured tool log cards (▣ tool_name + args preview)
- no-background activity indicator and meta bar for cleaner rendering

Startup behavior:
- default startup creates a fresh session
- explicit --resume <id> restores saved context
- added main_test.go for initializeSession coverage

main_test.go

@@ -0,0 +1,101 @@
+package main
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"agentu/internal/agent"
+	"agentu/internal/session"
+	"agentu/pkg/config"
+	"agentu/pkg/llm"
+)
+
+type startupProvider struct{}
+
+func (p *startupProvider) ChatStream(ctx context.Context, req llm.ChatRequest, emit func(llm.StreamEvent) error) error {
+	return emit(llm.StreamEvent{Content: "ok"})
+}
+
+func startupTestManager(t *testing.T) (*session.Manager, *agent.Agent) {
+	t.Helper()
+	cfg := &config.Config{
+		Providers: map[string]config.ProviderConfig{
+			"one": {
+				Name:          "one",
+				BaseURL:       "https://one.example.com",
+				APIKey:        "sk-test",
+				Model:         "model-a",
+				Models:        []string{"model-a"},
+				Thinking:      "none",
+				ThinkingParam: "thinking",
+			},
+		},
+	}
+	assistant := agent.New(agent.Options{
+		Provider:     &startupProvider{},
+		ProviderName: "one",
+		Model:        "model-a",
+	})
+	store, err := session.NewStore(filepath.Join(t.TempDir(), "sessions"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	manager, err := session.NewManager(cfg, assistant, nil, store)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return manager, assistant
+}
+
+func TestInitializeSessionStartsNewByDefault(t *testing.T) {
+	manager, assistant := startupTestManager(t)
+	if _, err := manager.NewSession(); err != nil {
+		t.Fatalf("new session: %v", err)
+	}
+	savedID := manager.CurrentSessionID()
+	assistant.SetMessages([]llm.Message{{Role: llm.RoleUser, Content: "old context"}})
+	if err := manager.Save(); err != nil {
+		t.Fatalf("save: %v", err)
+	}
+
+	if err := initializeSession(manager, ""); err != nil {
+		t.Fatalf("initialize: %v", err)
+	}
+	if manager.CurrentSessionID() == savedID {
+		t.Fatalf("default startup resumed latest session %s", savedID)
+	}
+	for _, msg := range assistant.Messages() {
+		if msg.Content == "old context" {
+			t.Fatal("default startup carried old context into new session")
+		}
+	}
+}
+
+func TestInitializeSessionResumesExplicitID(t *testing.T) {
+	manager, assistant := startupTestManager(t)
+	if _, err := manager.NewSession(); err != nil {
+		t.Fatalf("new session: %v", err)
+	}
+	savedID := manager.CurrentSessionID()
+	assistant.SetMessages([]llm.Message{{Role: llm.RoleUser, Content: "old context"}})
+	if err := manager.Save(); err != nil {
+		t.Fatalf("save: %v", err)
+	}
+
+	if err := initializeSession(manager, savedID); err != nil {
+		t.Fatalf("initialize: %v", err)
+	}
+	if manager.CurrentSessionID() != savedID {
+		t.Fatalf("expected resumed session %s, got %s", savedID, manager.CurrentSessionID())
+	}
+	found := false
+	for _, msg := range assistant.Messages() {
+		if msg.Content == "old context" {
+			found = true
+		}
+	}
+	if !found {
+		t.Fatal("explicit resume did not restore saved context")
+	}
+}