fix: add cmd/five/main.go to repo (was excluded by .gitignore)

- Changed .gitignore: "five" → "/five" to only ignore root binary
- cmd/five/main.go (702 LOC): Five CLI entry point (run, build, gen, debug, frb)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -4,7 +4,7 @@
 *.dll
 *.so
 *.dylib
-five
+/five
 
 # Test binary
 *.test