With Obsidian just added to that stack, a small decision note might help keep your creative calls intact across Claude/Codex sessions: what you've decided, why it matters, and what is still open. An agent's implementation suggestion can stay in a separate note until you choose to adopt it, so a convenient coding shortcut doesn't quietly become worldbuilding canon.
For the system-heavy side, you could connect each decision to one tiny Unity check: the starting state, one action, and the result you expect. A short editor clip can then show that exact check, with the note linking back to it. That would give those progress clips a second use when a later change breaks an interaction. Are you planning to keep Obsidian as the source of truth for lore, or will the GDD fill that role?