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>

mocks/path.go

@@ -0,0 +1,23 @@
+// Package mocks provides test fixture data and helpers for unit tests.
+package mocks
+
+import (
+	"fmt"
+	"path/filepath"
+	"runtime"
+)
+
+// Path returns the absolute path to the specified mock file within
+// the mocks/ directory.
+func Path(filename string) string {
+	_, thisfile, _, _ := runtime.Caller(0)
+
+	file, err := filepath.Abs(
+		filepath.Join(filepath.Dir(thisfile), filename),
+	)
+	if err != nil {
+		panic(fmt.Errorf("failed to resolve mock path: %v", err))
+	}
+
+	return file
+}