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/cheatpath/filter.go

@@ -5,15 +5,15 @@ import (
 )
 
 // Filter filters all cheatpaths that are not named `name`
-func Filter(paths []Cheatpath, name string) ([]Cheatpath, error) {
+func Filter(paths []Path, name string) ([]Path, error) {
 
 	// if a path of the given name exists, return it
 	for _, path := range paths {
 		if path.Name == name {
-			return []Cheatpath{path}, nil
+			return []Path{path}, nil
 		}
 	}
 
 	// otherwise, return an error
-	return []Cheatpath{}, fmt.Errorf("cheatpath does not exist: %s", name)
+	return []Path{}, fmt.Errorf("cheatpath does not exist: %s", name)
 }