Fix lint issues and improve test suite resilience

- compose.matrix.yml: add trailing newline
- compose.ftp-222*.yml: fix YAML indentation (8->6 spaces)
- entrypoint.sh.tmpl: fix SC2198 ([ -n "$@" ] -> [ $# -gt 0 ])
- All .tmpl files: migrate from {{ env }} to {{ getenv }} for gomplate v5 compat
- uploads.ini.tmpl: add {{- / -}} whitespace trimming, fix double-space typo
- tests/run.sh: only run ShellCheck on *.sh.tmpl files
- tests/test_shell.sh: gracefully skip if shellcheck not installed
- tests/test_templates.sh: remove dead render() function,
  gracefully skip if gomplate not found, use set -a/. for env sourcing
- tests/test_compose_config.sh: validate override files combined
  with compose.yml, skip partial snippets needing more context
- README.md: add test instructions with brew install

tests/run.sh

@@ -19,7 +19,7 @@ done < <(find "$ROOT/.." -maxdepth 1 -name 'compose*.yml' | sort)
 SHELL_TMPL_FILES=()
 while IFS= read -r f; do
     SHELL_TMPL_FILES+=("$f")
-done < <(find "$ROOT/.." -maxdepth 1 -name '*.tmpl' | sort)
+done < <(find "$ROOT/.." -maxdepth 1 -name '*.sh.tmpl' | sort)
 
 # Track total failures across all test suites
 failures=0