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/display/write.go

@@ -24,7 +24,7 @@ func Write(out string, conf config.Config) {
 
 // writeToPager writes output through a pager command
 func writeToPager(out string, conf config.Config) {
-	parts := strings.Split(conf.Pager, " ")
+	parts := strings.Fields(conf.Pager)
 	pager := parts[0]
 	args := parts[1:]