fix: harden file/directory permissions to owner-only (C5+H9+M9+M10)

Add mode 0o700 to all mkdirSync calls for state/session directories.
Add mode 0o600 to all writeFileSync calls for session.json, chat.jsonl,
and log files. Add umask 077 to setup script. Prevents auth tokens, chat
history, and browser logs from being world-readable on multi-user systems.
Closes C5, H9, M9, M10 from security audit #783.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

setup

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 # gstack setup — build browser binary + register skills with Claude Code / Codex
 set -e
+umask 077  # Restrict new files to owner-only (0o600 files, 0o700 dirs)
 
 if ! command -v bun >/dev/null 2>&1; then
   echo "Error: bun is required but not installed." >&2