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/sheet/tagged.go

@@ -1,15 +1,8 @@
 package sheet
 
+import "slices"
+
 // Tagged returns true if a sheet was tagged with `needle`
 func (s *Sheet) Tagged(needle string) bool {
-
-	// if any of the tags match `needle`, return `true`
-	for _, tag := range s.Tags {
-		if tag == needle {
-			return true
-		}
-	}
-
-	// otherwise, return `false`
-	return false
+	return slices.Contains(s.Tags, needle)
 }