Cross-platform gstack enforcement hook

Design decisions

• Required mode generates .claude/hooks/check-gstack.cjs. The explicit CommonJS extension is unaffected by a consumer repository's package.json module type.
• The one registered command launches Node 18+ and resolves CLAUDE_PROJECT_DIR inside JavaScript through process.env. It contains no Bash, PowerShell, or cmd environment expansion.
• The launcher validates CLAUDE_PROJECT_DIR before calling path.join. Missing or empty project context emits a structured PreToolUse denial, writes a clear reason to stderr, and exits zero instead of surfacing a hook execution error.
• Path resolution and module loading run inside the same try/catch. A nonexistent project, an existing project without the generated hook, or any other load failure returns the same concise denial without exposing a stack trace or the full project path.
• The generated hook resolves the user home in HOME, USERPROFILE, then os.homedir() order so POSIX, Git Bash, PowerShell, and cmd runners use the expected global install root.
• The hook contract follows docs/spikes/claude-code-hook-mutation.md: pass-through writes {}; denial writes a hookSpecificOutput object for PreToolUse with permissionDecision: "deny".
• Missing or unreadable gstack is an intentional denial with exit code zero. Install instructions are present in stderr and permissionDecisionReason, so the runner does not report a hook execution error.
• Rerunning required mode replaces a matching legacy check-gstack.sh registration and removes duplicate matching registrations.

Deviations

None. The implementation uses the requested Node launcher, one hook handler, and the repository's documented hook output schema.

Tradeoffs

• The generated project now requires Node 18+ at hook execution time. This removes shell selection ambiguity and matches the requested runtime floor.
• The generated hook is invoked through Node rather than as an executable file, so POSIX executable mode bits are no longer part of the contract.

Open questions

None.