read all AGENTS.md up to git root

[nornagon-openai]

This updates our logic for AGENTS.md to match documented behavior, which is to read all AGENTS.md files from cwd up to git root.

[bolinfest]

I thought we discussed letting the model decide to do this (as is true today)?

[nornagon-openai]

@bolinfest I don't think that's the case today?

codex/codex-rs/core/src/codex.rs

Lines 504 to 509 in 250ae37

	// record the initial user instructions and environment context,
	// regardless of whether we restored items.
	let mut conversation_items = Vec::<ResponseItem>::with_capacity(2);
	if let Some(user_instructions) = turn_context.user_instructions.as_deref() {
	conversation_items.push(Prompt::format_user_instructions_message(user_instructions));
	}

[nornagon-openai]

Happy to refactor this further as we evolve our thinking about how the model should access the AGENTS.md, but for now I think this is an improvement over the current state.