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/search_fuzz_test.go

@@ -122,69 +122,3 @@ func FuzzSearchRegex(f *testing.F) {
 		}
 	})
 }
-
-// FuzzSearchCatastrophicBacktracking specifically tests for regex patterns
-// that could cause performance issues
-func FuzzSearchCatastrophicBacktracking(f *testing.F) {
-	// Seed with patterns known to potentially cause issues
-	f.Add("a", 10, 5)
-	f.Add("x", 20, 3)
-
-	f.Fuzz(func(t *testing.T, char string, repeats int, groups int) {
-		// Limit the size to avoid memory issues in the test
-		if repeats > 30 || repeats < 0 || groups > 10 || groups < 0 || len(char) > 5 {
-			t.Skip("Skipping invalid or overly large test case")
-		}
-
-		// Construct patterns that might cause backtracking
-		patterns := []string{
-			strings.Repeat(char, repeats),
-			"(" + char + "+)+",
-			"(" + char + "*)*",
-			"(" + char + "|" + char + ")*",
-		}
-
-		// Add nested groups
-		if groups > 0 && groups < 10 {
-			nested := char
-			for i := 0; i < groups; i++ {
-				nested = "(" + nested + ")+"
-			}
-			patterns = append(patterns, nested)
-		}
-
-		// Test text that might trigger backtracking
-		testText := strings.Repeat(char, repeats) + "x"
-
-		for _, pattern := range patterns {
-			// Try to compile the pattern
-			reg, err := regexp.Compile(pattern)
-			if err != nil {
-				// Invalid pattern, skip
-				continue
-			}
-
-			// Test with timeout
-			done := make(chan bool, 1)
-
-			go func() {
-				defer func() {
-					if r := recover(); r != nil {
-						t.Errorf("Search panicked with backtracking pattern %q: %v", pattern, r)
-					}
-					done <- true
-				}()
-
-				sheet := Sheet{Text: testText}
-				_ = sheet.Search(reg)
-			}()
-
-			select {
-			case <-done:
-				// Completed successfully
-			case <-time.After(50 * time.Millisecond):
-				t.Logf("Warning: potential backtracking issue with pattern %q (completed slowly)", pattern)
-			}
-		}
-	})
-}