compact: retain newest ~20% of context; refactor startup into internal/app

- Replace fixed 6-message compact retention with ratio-based logic:
  retain ~20% of compactable estimated tokens, with a 6-message floor
  and user-turn boundary alignment
- Add compactRetainedTokenBudget, compactRecentStart, compactTurnBoundary
  helpers in internal/agent/agent.go
- Add TestAgentCompactKeepsTwentyPercentRecentContext
- Move startup/session/repl/bootstrap logic from root into internal/app
  so main.go is a thin binary entry point (15 lines)
- Update /compact docs in README.md

internal/app/bootstrap.go

@@ -0,0 +1,38 @@
+package app
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"agentu/pkg/config"
+)
+
+const defaultConfigContent = `# agentu configuration
+# Replace the placeholder values below with your actual provider settings.
+# See agentu.example.yaml for the full reference.
+providers:
+  default:
+    base_url: https://api.openai.com
+    api_key: YOUR_API_KEY_HERE
+    models:
+      - id: gpt-4o
+        context: 128000
+      - id: gpt-4o-mini
+        context: 128000
+`
+
+func bootstrapConfig(configPath string) error {
+	resolved, err := config.ResolvePath(configPath)
+	if err != nil {
+		return fmt.Errorf("resolve config path: %w", err)
+	}
+	dir := filepath.Dir(resolved)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return fmt.Errorf("create config directory: %w", err)
+	}
+	if err := os.WriteFile(resolved, []byte(defaultConfigContent), 0o644); err != nil {
+		return fmt.Errorf("write default config: %w", err)
+	}
+	return nil
+}