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/agent/agent_test.go

@@ -145,6 +145,9 @@ func TestAgentCompact(t *testing.T) {
 	if messages[len(messages)-1].Content != "recent-answer-2" {
 		t.Fatalf("last recent message = %#v", messages[len(messages)-1])
 	}
+	if messages[2].Content != "recent-user-0" {
+		t.Fatalf("first retained recent message = %#v", messages[2])
+	}
 	if usage := a.LastUsage(); usage == nil || usage.TotalTokens != 12 {
 		t.Fatalf("usage = %#v", usage)
 	}
@@ -217,6 +220,48 @@ func longConversation() []llm.Message {
 	return messages
 }
 
+func manyTurnConversation(turns int) []llm.Message {
+	messages := []llm.Message{{Role: llm.RoleSystem, Content: "system"}}
+	for i := 0; i < turns; i++ {
+		messages = append(messages,
+			llm.Message{Role: llm.RoleUser, Content: fmt.Sprintf("user-%02d", i)},
+			llm.Message{Role: llm.RoleAssistant, Content: fmt.Sprintf("assistant-%02d", i)},
+		)
+	}
+	return messages
+}
+
+func TestAgentCompactKeepsTwentyPercentRecentContext(t *testing.T) {
+	provider := &compactProvider{}
+	a := New(Options{Provider: provider, Model: "test", SystemPrompt: "system"})
+	a.SetMessages(manyTurnConversation(25))
+
+	if err := a.Compact(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+
+	messages := a.Messages()
+	// 1 system + 1 summary + 10 retained recent messages (5 user/assistant turns)
+	if len(messages) != 12 {
+		t.Fatalf("messages = %d, want 12; contents: %v", len(messages), joinMessageContents(messages))
+	}
+	if provider.summaryCalls != 1 {
+		t.Fatalf("summaryCalls = %d, want 1", provider.summaryCalls)
+	}
+	if messages[2].Content != "user-20" {
+		t.Fatalf("first retained message = %#v, want user-20", messages[2])
+	}
+	if messages[len(messages)-1].Content != "assistant-24" {
+		t.Fatalf("last retained message = %#v, want assistant-24", messages[len(messages)-1])
+	}
+	if !strings.Contains(provider.lastSummary, "user-19") {
+		t.Fatalf("summary should contain user-19 (part of summarized 80%%): %q", provider.lastSummary)
+	}
+	if strings.Contains(provider.lastSummary, "user-20") {
+		t.Fatalf("summary should not contain user-20 (part of retained 20%%): %q", provider.lastSummary)
+	}
+}
+
 func joinMessageContents(messages []llm.Message) string {
 	var b strings.Builder
 	for _, msg := range messages {