chore: housekeeping and refactoring (bump to 4.7.1)

- Remove unused parameters, dead files, and inaccurate doc.go files
- Extract shared helpers, eliminate duplication
- Rename cheatpath.Cheatpath to cheatpath.Path
- Optimize filesystem walks (WalkDir, skip .git)
- Move sheet name validation to sheet.Validate
- Move integration tests to test/integration/
- Consolidate internal/mock into mocks/
- Move fuzz.sh to test/
- Inline loadSheets helper into command callers
- Extract config.New into its own file
- Fix stale references in HACKING.md and CLAUDE.md
- Restore plan9 build target
- Remove redundant and low-value tests
- Clean up project documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/installer/prompt_test.go

@@ -2,7 +2,6 @@ package installer
 
 import (
 	"bytes"
-	"fmt"
 	"io"
 	"os"
 	"strings"
@@ -158,23 +157,3 @@ func TestPromptError(t *testing.T) {
 		t.Errorf("expected 'failed to prompt' error, got: %v", err)
 	}
 }
-
-// TestPromptIntegration provides a simple integration test
-func TestPromptIntegration(t *testing.T) {
-	// This demonstrates how the prompt would be used in practice
-	// It's skipped by default since it requires actual user input
-	if os.Getenv("TEST_INTERACTIVE") != "1" {
-		t.Skip("Skipping interactive test - set TEST_INTERACTIVE=1 to run")
-	}
-
-	fmt.Println("\n=== Interactive Prompt Test ===")
-	fmt.Println("You will be prompted to answer a question.")
-	fmt.Println("Try different inputs: y, n, Y, N, empty (just press Enter)")
-
-	result, err := Prompt("Would you like to continue? [Y/n]", true)
-	if err != nil {
-		t.Fatalf("Prompt failed: %v", err)
-	}
-
-	fmt.Printf("You answered: %v\n", result)
-}