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/session_startup.go

@@ -0,0 +1,38 @@
+package app
+
+import (
+	"fmt"
+	"os"
+
+	"agentu/internal/session"
+)
+
+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 printExitSession(m *session.Manager) {
+	if m == nil {
+		return
+	}
+	_ = m.Save()
+	id := m.CurrentSessionID()
+	name := m.CurrentSessionName()
+	if id != "" {
+		if name != "" && name != id {
+			fmt.Fprintf(os.Stderr, "session: %s (%s)\n", id, name)
+		} else {
+			fmt.Fprintf(os.Stderr, "session: %s\n", id)
+		}
+		fmt.Fprintf(os.Stderr, "resume:  agentu --resume %s\n", id)
+	}
+}