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/run_test.go

@@ -1,7 +1,6 @@
 package installer
 
 import (
-	"fmt"
 	"io"
 	"os"
 	"path/filepath"
@@ -245,10 +244,10 @@ cheatpaths:
 	if strings.Contains(contentStr, "PERSONAL_PATH") {
 		t.Error("PERSONAL_PATH was not replaced")
 	}
-	if strings.Contains(contentStr, "EDITOR_PATH") && !strings.Contains(contentStr, fmt.Sprintf("editor: %s", "")) {
+	if strings.Contains(contentStr, "EDITOR_PATH") {
 		t.Error("EDITOR_PATH was not replaced")
 	}
-	if strings.Contains(contentStr, "PAGER_PATH") && !strings.Contains(contentStr, fmt.Sprintf("pager: %s", "")) {
+	if strings.Contains(contentStr, "PAGER_PATH") {
 		t.Error("PAGER_PATH was not replaced")
 	}
 	if strings.Contains(contentStr, "WORK_PATH") {