Addresses Codex review on PR #947: the platform-var detector still treated
ANY \`\${VAR:-...}\` shell default as graceful, so a future skill could
reintroduce the exact #943 regression — \`bash "\${CLAUDE_SKILL_DIR:-.}/scripts/x"\`,
which on native Codex/Gemini falls back to \`./scripts/...\` in the project CWD
and silently misses the bundled script — and the convention test would pass.
Exclude skill-directory vars (CLAUDE_SKILL_DIR, CLAUDE_PLUGIN_ROOT) from the
\`:-\` graceful path: for those, only an existence guard (\`[ -f "\${VAR}/path" ]\`)
or an explicit acknowledgment counts. Ordinary vars keep the shell-default
fallback. ce-worktree still uses the legacy \`:-.\` form on this branch, so its
acknowledgment reason is updated to name it a known laggard tracked by #946/#948.
Adds regression guards asserting \`\${CLAUDE_SKILL_DIR:-.}\` is reported.
Trevin Chow
fa990c4ca0