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>

test/integration/helpers_test.go

@@ -0,0 +1,30 @@
+package integration
+
+import (
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+// repoRoot returns the absolute path to the repository root.
+// It derives this from the known location of this source file
+// (test/integration/) relative to the repo root.
+func repoRoot(t *testing.T) string {
+	t.Helper()
+	_, file, _, ok := runtime.Caller(0)
+	if !ok {
+		t.Fatal("failed to determine repo root via runtime.Caller")
+	}
+	// file is <repo>/test/integration/helpers_test.go → go up two dirs
+	return filepath.Dir(filepath.Dir(filepath.Dir(file)))
+}
+
+// repoRootBench is the same as repoRoot but for use in benchmarks.
+func repoRootBench(b *testing.B) string {
+	b.Helper()
+	_, file, _, ok := runtime.Caller(0)
+	if !ok {
+		b.Fatal("failed to determine repo root via runtime.Caller")
+	}
+	return filepath.Dir(filepath.Dir(filepath.Dir(file)))
+}