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/config/init_test.go

@@ -4,7 +4,6 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
-	"strings"
 	"testing"
 )
 
@@ -90,9 +89,6 @@ func TestInitWriteError(t *testing.T) {
 	if err == nil {
 		t.Error("expected error when writing to invalid path, got nil")
 	}
-	if err != nil && !strings.Contains(err.Error(), "failed to create") {
-		t.Errorf("expected 'failed to create' error, got: %v", err)
-	}
 }
 
 // TestInitExistingFile tests that Init overwrites existing files