fix: refactor shell executable assignment to use class constant (#168)

Signed-off-by: Frost Ming <me@frostming.com>

Author: frostming

src/bub/builtin/shell_manager.py

@@ -31,6 +31,8 @@ def status(self) -> str:
 
 
 class ShellManager:
+    SHELL = shutil.which("bash") or shutil.which("sh") if os.name != "nt" else None
+
     def __init__(self) -> None:
         self._shells: dict[str, ManagedShell] = {}
 
@@ -40,7 +42,7 @@ async def start(self, *, cmd: str, cwd: str | None) -> ManagedShell:
             cwd=cwd,
             stdout=asyncio.subprocess.PIPE,
             stderr=asyncio.subprocess.PIPE,
-            executable=(shutil.which("bash") or shutil.which("sh")) if os.name != "nt" else None,
+            executable=self.SHELL,
         )
         shell = ManagedShell(shell_id=f"bash-{uuid.uuid4().hex[:8]}", cmd=cmd, cwd=cwd, process=process)
         shell.read_tasks.extend([