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>

CONTRIBUTING.md

@@ -1,5 +1,4 @@
-Contributing
-============
+# Contributing
 
 Thank you for your interest in `cheat`.
 
@@ -11,4 +10,8 @@ Bug reports are still welcome. If you've found a bug, please open an issue in
 the [issue tracker][issues]. Before doing so, please search through the
 existing open issues to make sure it hasn't already been reported.
 
+Feature requests may be filed, but are unlikely to be implemented. The project
+is now mature and the maintainer considers its feature set to be essentially
+complete.
+
 [issues]: https://github.com/cheat/cheat/issues