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>

adr/001-path-traversal-protection.md

@@ -38,10 +38,10 @@ The validation is performed at the application layer before any file operations
 
 ### Validation Function
 
-The validation is implemented in `internal/cheatpath/validate.go`:
+The validation is implemented in `internal/sheet/validate.go`:
 
 ```go
-func ValidateSheetName(name string) error {
+func Validate(name string) error {
     // Reject empty names
     if name == "" {
         return fmt.Errorf("cheatsheet name cannot be empty")
@@ -133,7 +133,7 @@ The following patterns are explicitly allowed:
 
 Comprehensive tests ensure the validation works correctly:
 
-1. **Unit tests** (`internal/cheatpath/validate_test.go`) verify the validation logic
+1. **Unit tests** (`internal/sheet/validate_test.go`) verify the validation logic
 2. **Integration tests** verify the actual binary blocks malicious inputs
 3. **No system files are accessed** during testing - all tests use isolated directories