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/sheet_test.go

@@ -4,7 +4,7 @@ import (
 	"reflect"
 	"testing"
 
-	"github.com/cheat/cheat/internal/mock"
+	"github.com/cheat/cheat/mocks"
 )
 
 // TestSheetSuccess asserts that sheets initialize properly
@@ -14,7 +14,7 @@ func TestSheetSuccess(t *testing.T) {
 	sheet, err := New(
 		"foo",
 		"community",
-		mock.Path("sheet/foo"),
+		mocks.Path("sheet/foo"),
 		[]string{"alpha", "bravo"},
 		false,
 	)
@@ -27,10 +27,10 @@ func TestSheetSuccess(t *testing.T) {
 		t.Errorf("failed to init title: want: foo, got: %s", sheet.Title)
 	}
 
-	if sheet.Path != mock.Path("sheet/foo") {
+	if sheet.Path != mocks.Path("sheet/foo") {
 		t.Errorf(
 			"failed to init path: want: %s, got: %s",
-			mock.Path("sheet/foo"),
+			mocks.Path("sheet/foo"),
 			sheet.Path,
 		)
 	}
@@ -63,7 +63,7 @@ func TestSheetFailure(t *testing.T) {
 	_, err := New(
 		"foo",
 		"community",
-		mock.Path("/does-not-exist"),
+		mocks.Path("/does-not-exist"),
 		[]string{"alpha", "bravo"},
 		false,
 	)
@@ -80,7 +80,7 @@ func TestSheetFrontMatterFailure(t *testing.T) {
 	_, err := New(
 		"foo",
 		"community",
-		mock.Path("sheet/bad-fm"),
+		mocks.Path("sheet/bad-fm"),
 		[]string{"alpha", "bravo"},
 		false,
 	)