fix: restore shellcheck/shfmt to [tools] for mise-managed installation

'system' is not a valid mise version. The correct approach is to pin
versions in [tools] so mise installs and manages them. In environments
where they are already available (e.g. this sandbox, devcontainers),
set MISE_DISABLE_TOOLS=shellcheck,shfmt to skip installation and use
the PATH versions instead.

Author: claude

mise.toml

@@ -18,6 +18,8 @@ _.path = ["./bin"]
 [tools]
 flutter = "3.38.3"
 node = "22"        # For http_server and Playwright e2e tests
+shellcheck = "0.9.0"
+shfmt = "3.8.0"
 
 [deps.flutter]
 auto = true