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/writeable.go

@@ -4,8 +4,8 @@ import (
 	"fmt"
 )
 
-// Writeable returns a writeable Cheatpath
-func Writeable(cheatpaths []Cheatpath) (Cheatpath, error) {
+// Writeable returns a writeable Path
+func Writeable(cheatpaths []Path) (Path, error) {
 
 	// iterate backwards over the cheatpaths
 	// NB: we're going backwards because we assume that the most "local"
@@ -18,5 +18,5 @@ func Writeable(cheatpaths []Cheatpath) (Cheatpath, error) {
 	}
 
 	// otherwise, return an error
-	return Cheatpath{}, fmt.Errorf("no writeable cheatpaths found")
+	return Path{}, fmt.Errorf("no writeable cheatpaths found")
 }