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/sheet/validate_test.go

@@ -0,0 +1,113 @@
+package sheet
+
+import (
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestValidate(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		wantErr bool
+		errMsg  string
+	}{
+		// Valid names
+		{
+			name:    "simple name",
+			input:   "docker",
+			wantErr: false,
+		},
+		{
+			name:    "name with slash",
+			input:   "docker/compose",
+			wantErr: false,
+		},
+		{
+			name:    "name with multiple slashes",
+			input:   "lang/go/slice",
+			wantErr: false,
+		},
+		{
+			name:    "name with dash and underscore",
+			input:   "my-cheat_sheet",
+			wantErr: false,
+		},
+		// Invalid names
+		{
+			name:    "empty name",
+			input:   "",
+			wantErr: true,
+			errMsg:  "empty",
+		},
+		{
+			name:    "parent directory traversal",
+			input:   "../etc/passwd",
+			wantErr: true,
+			errMsg:  "'..'",
+		},
+		{
+			name:    "complex traversal",
+			input:   "foo/../../etc/passwd",
+			wantErr: true,
+			errMsg:  "'..'",
+		},
+		{
+			name:    "absolute path unix",
+			input:   "/etc/passwd",
+			wantErr: runtime.GOOS != "windows", // /etc/passwd is not absolute on Windows
+			errMsg:  "absolute",
+		},
+		{
+			name:    "absolute path windows",
+			input:   `C:\evil`,
+			wantErr: runtime.GOOS == "windows", // C:\evil is not absolute on Unix
+			errMsg:  "absolute",
+		},
+		{
+			name:    "home directory",
+			input:   "~/secrets",
+			wantErr: true,
+			errMsg:  "'~'",
+		},
+		{
+			name:    "just dots",
+			input:   "..",
+			wantErr: true,
+			errMsg:  "'..'",
+		},
+		{
+			name:    "hidden file not allowed",
+			input:   ".hidden",
+			wantErr: true,
+			errMsg:  "cannot start with '.'",
+		},
+		{
+			name:    "current dir is ok",
+			input:   "./current",
+			wantErr: false,
+		},
+		{
+			name:    "nested hidden file not allowed",
+			input:   "config/.gitignore",
+			wantErr: true,
+			errMsg:  "cannot start with '.'",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := Validate(tt.input)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Validate(%q) error = %v, wantErr %v", tt.input, err, tt.wantErr)
+				return
+			}
+			if err != nil && tt.errMsg != "" {
+				if !strings.Contains(err.Error(), tt.errMsg) {
+					t.Errorf("Validate(%q) error = %v, want error containing %q", tt.input, err, tt.errMsg)
+				}
+			}
+		})
+	}
+}