Design a conservative Codex notification-hook installer

[ajhcs]

Design an installer or helper that adds Codex Edge Glow to config.toml without destroying an existing notifier. Ambiguous input must stop for user review.

Acceptance criteria

• No existing notify command is overwritten silently.
• Existing configuration is backed up before mutation.
• Direct and chained configurations have tests covering spaces and quotes.
• A dry-run displays the proposed change.
• Uninstall can restore the previous value.
• Threat boundaries and failure recovery are documented.