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_fuzz_test.go

@@ -0,0 +1,169 @@
+package sheet
+
+import (
+	"strings"
+	"testing"
+	"unicode/utf8"
+)
+
+// FuzzValidate tests the Validate function with fuzzing
+// to ensure it properly prevents path traversal and other security issues
+func FuzzValidate(f *testing.F) {
+	// Add seed corpus with various valid and malicious inputs
+	// Valid names
+	f.Add("docker")
+	f.Add("docker/compose")
+	f.Add("lang/go/slice")
+	f.Add("my-cheat_sheet")
+	f.Add("file.txt")
+	f.Add("a")
+	f.Add("123")
+
+	// Path traversal attempts
+	f.Add("..")
+	f.Add("../etc/passwd")
+	f.Add("foo/../bar")
+	f.Add("foo/../../etc/passwd")
+	f.Add("..\\windows\\system32")
+	f.Add("foo\\..\\..\\windows")
+
+	// Encoded traversal attempts
+	f.Add("%2e%2e")
+	f.Add("%2e%2e%2f")
+	f.Add("..%2f")
+	f.Add("%2e.")
+	f.Add(".%2e")
+	f.Add("\x2e\x2e")
+	f.Add("\\x2e\\x2e")
+
+	// Unicode and special characters
+	f.Add("€test")
+	f.Add("test€")
+	f.Add("中文")
+	f.Add("🎉emoji")
+	f.Add("\x00null")
+	f.Add("test\x00null")
+	f.Add("\nnewline")
+	f.Add("test\ttab")
+
+	// Absolute paths
+	f.Add("/etc/passwd")
+	f.Add("C:\\Windows\\System32")
+	f.Add("\\\\server\\share")
+	f.Add("//server/share")
+
+	// Home directory
+	f.Add("~")
+	f.Add("~/config")
+	f.Add("~user/file")
+
+	// Hidden files
+	f.Add(".hidden")
+	f.Add("dir/.hidden")
+	f.Add(".git/config")
+
+	// Edge cases
+	f.Add("")
+	f.Add(" ")
+	f.Add("  ")
+	f.Add("\t")
+	f.Add(".")
+	f.Add("./")
+	f.Add("./file")
+	f.Add(".../")
+	f.Add("...")
+	f.Add("....")
+
+	// Very long names
+	f.Add(strings.Repeat("a", 255))
+	f.Add(strings.Repeat("a/", 100) + "file")
+	f.Add(strings.Repeat("../", 50) + "etc/passwd")
+
+	f.Fuzz(func(t *testing.T, input string) {
+		// The function should never panic
+		func() {
+			defer func() {
+				if r := recover(); r != nil {
+					t.Errorf("Validate panicked with input %q: %v", input, r)
+				}
+			}()
+
+			err := Validate(input)
+
+			// Security invariants that must always hold
+			if err == nil {
+				// If validation passed, verify security properties
+
+				// Should not contain ".." for path traversal
+				if strings.Contains(input, "..") {
+					t.Errorf("validation passed but input contains '..': %q", input)
+				}
+
+				// Should not be empty
+				if input == "" {
+					t.Error("validation passed for empty input")
+				}
+
+				// Should not start with ~ (home directory)
+				if strings.HasPrefix(input, "~") {
+					t.Errorf("validation passed but input starts with '~': %q", input)
+				}
+
+				// Base filename should not start with .
+				parts := strings.Split(input, "/")
+				if len(parts) > 0 {
+					lastPart := parts[len(parts)-1]
+					if strings.HasPrefix(lastPart, ".") && lastPart != "." {
+						t.Errorf("validation passed but filename starts with '.': %q", input)
+					}
+				}
+
+				// Additional check: result should be valid UTF-8
+				if !utf8.ValidString(input) {
+					// While the function doesn't explicitly check this,
+					// we want to ensure it handles invalid UTF-8 gracefully
+					t.Logf("validation passed for invalid UTF-8: %q", input)
+				}
+			}
+		}()
+	})
+}
+
+// FuzzValidatePathTraversal specifically targets path traversal bypasses
+func FuzzValidatePathTraversal(f *testing.F) {
+	// Seed corpus focusing on path traversal variations
+	f.Add("..", "/", "")
+	f.Add("", "..", "/")
+	f.Add("a", "b", "c")
+
+	f.Fuzz(func(t *testing.T, prefix string, middle string, suffix string) {
+		// Construct various path traversal attempts
+		inputs := []string{
+			prefix + ".." + suffix,
+			prefix + "/.." + suffix,
+			prefix + "\\.." + suffix,
+			prefix + middle + ".." + suffix,
+			prefix + "../" + middle + suffix,
+			prefix + "..%2f" + suffix,
+			prefix + "%2e%2e" + suffix,
+			prefix + "%2e%2e%2f" + suffix,
+		}
+
+		for _, input := range inputs {
+			func() {
+				defer func() {
+					if r := recover(); r != nil {
+						t.Errorf("Validate panicked with constructed input %q: %v", input, r)
+					}
+				}()
+
+				err := Validate(input)
+
+				// If the input contains literal "..", it must be rejected
+				if strings.Contains(input, "..") && err == nil {
+					t.Errorf("validation incorrectly passed for input containing '..': %q", input)
+				}
+			}()
+		}
+	})
+}