chore: update bin scripts — rename references from .sh to plain names

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bin/README.md

@@ -2,7 +2,7 @@
 
 Personal shell scripts, available system-wide via `~/scripts` in `PATH`.
 
-## stacks.sh
+## stacks
 
 Docker Compose stack manager for `~/stacks`. Set `STACKS_DIR` to override the default location.
 
@@ -60,7 +60,7 @@ stacks completion zsh --static --install  # static (bakes current list in)
 
 **Global flags:** `--dry-run`, `--parallel`, `--verbose`, `--quiet`
 
-## arty.sh
+## arty
 
 Bash library repository and release management system. Run inside a directory with an `arty.yml`.