fix(config-store): write the global config with 0600 permissions

[davidfirst]

The global config holds the user's user.token but is written without a mode option, so it inherits the default umask (often 0644). Tighten to 0600 (matches AWS CLI / gcloud / kubectl / gh).

Same shape as server-port.txt hardening from #10344 — pass mode: 0o600 on write and explicitly chmod after, since Node only honors the mode option on file creation.

[Copilot]

Pull request overview

This PR hardens the on-disk permissions for Bit’s global config file (config.json under the global cache dir), which can contain sensitive values like user.token. It ensures the file is written with owner-only permissions (0600) instead of inheriting potentially permissive defaults from the system umask.

Changes:

• Introduce a CONFIG_FILE_MODE = 0o600 constant for the global config file.
• Update GlobalConfig.write() to write with { mode: 0o600 } and then explicitly chmod to 0600 (to cover pre-existing files).
• Update GlobalConfig.writeSync() similarly (write with mode + chmodSync).