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.go

@@ -13,11 +13,11 @@ import (
 	"strings"
 
 	"agentu/internal/agent"
-	"agentu/pkg/config"
-	"agentu/pkg/llm"
 	"agentu/internal/session"
 	"agentu/internal/tools"
 	"agentu/internal/tui"
+	"agentu/pkg/config"
+	"agentu/pkg/llm"
 )
 
 const defaultConfigContent = `# agentu configuration
@@ -112,15 +112,8 @@ func run() error {
 		return err
 	}
 
-	// Resume session
-	if *resumeID != "" {
-		if err := modelManager.Resume(*resumeID); err != nil {
-			return fmt.Errorf("resume session: %w", err)
-		}
-	} else {
-		if err := modelManager.ResumeLatest(); err != nil {
-			return fmt.Errorf("resume latest session: %w", err)
-		}
+	if err := initializeSession(modelManager, *resumeID); err != nil {
+		return err
 	}
 
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
@@ -141,6 +134,19 @@ func run() error {
 	return repl(ctx, assistant, modelManager)
 }
 
+func initializeSession(modelManager *session.Manager, resumeID string) error {
+	if resumeID != "" {
+		if err := modelManager.Resume(resumeID); err != nil {
+			return fmt.Errorf("resume session: %w", err)
+		}
+		return nil
+	}
+	if _, err := modelManager.NewSession(); err != nil {
+		return fmt.Errorf("start new session: %w", err)
+	}
+	return nil
+}
+
 func bootstrapConfig(configPath string) error {
 	resolved, err := config.ResolvePath(configPath)
 	if err != nil {