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/parse_test.go

@@ -38,7 +38,7 @@ To foo the bar: baz`
 		t.Errorf("failed to parse tags: want: %s, got: %s", want, fm.Tags[0])
 	}
 	if len(fm.Tags) != 1 {
-		t.Errorf("failed to parse tags: want: len 0, got: len %d", len(fm.Tags))
+		t.Errorf("failed to parse tags: want: len 1, got: len %d", len(fm.Tags))
 	}
 }