`project_root_markers` is not honored for AGENTS.md discovery

What version of Codex CLI is running?

codex-cli 0.104.0

What subscription do you have?

Plus

Which model were you using?

gpt-5.3-codex

What platform is your computer?

Linux 6.18.7-arch1-1 x86_64 unknown

What terminal emulator and version are you using (if applicable)?

_No response_

What issue are you seeing?

<img width="1263" height="473" alt="Image" src="https://github.com/user-attachments/assets/ffd7587d-903d-4b4d-bf61-41dfd20508b2" />

The docs say Codex discovers project configuration — including both .codex/ layers and AGENTS.md — by walking up from the working directory until it reaches a project root, and that this root can be customized via project_root_markers (or disabled with []).

However, the CLI behavior does not match this documentation for AGENTS.md: even when project_root_markers is explicitly set to a parent marker (e.g., .codex-root), if the cwd contains a nested .git, Codex still resolves AGENTS.md as if the nested .git is a hard boundary and only loads the cwd AGENTS.md.

Observed situation:

/status reports:

Agents.md: AGENTS.md

when it should include the parent ../AGENTS.md as well.

What steps can reproduce the bug?

The following commands create a minimal repro directory structure like:

codex-bug-repro
├── AGENTS.md
├── .codex-root
└── dir1
    ├── AGENTS.md
    └── .git

mkdir -p codex-bug-repro/dir1/.git

touch codex-bug-repro/.codex-root
printf "PARENT AGENTS\n" > codex-bug-repro/AGENTS.md
printf "CHILD AGENTS\n"  > codex-bug-repro/dir1/AGENTS.md

cd codex-bug-repro/dir1

codex -c 'project_root_markers=[".codex-root"]'   # then run: /status
````

**result:** only `./AGENTS.md` is loaded, and `../AGENTS.md` is ignored.


### What is the expected behavior?

With `project_root_markers=[".codex-root"]` and the parent directory containing `.codex-root`, Codex should:

1. Treat the parent directory as the project root **even if** the cwd contains a nested `.git`.
2. Aggregate and load `AGENTS.md` along the path from project root → cwd (e.g., load both `../AGENTS.md` and `./AGENTS.md`).
3. Reflect this in `/status`, indicating that the parent `AGENTS.md` is included.

In short: `project_root_markers` should apply consistently to `AGENTS.md` discovery, matching the project discovery semantics used for `.codex/` project configuration.

### Additional information

### Root Cause

`codex-rs/core/src/project_doc.rs` resolves `AGENTS.md` by walking up to a **hard-coded `.git` root** and stopping there, so `project_root_markers` is ignored for agent-doc discovery in nested-repo setups.

### Proposed fix

Reuse the `project_root_markers` root-detection logic already used in `codex-rs/core/src/skills/loader.rs`:
- derive markers from merged **pre-project** config layers
- compute project root via markers
- aggregate `AGENTS.md` from `project_root → cwd`, plus a regression test for nested `.git`

**Happy to submit a PR if invited.**

### Related

- Docs: [Advanced Configuration](https://developers.openai.com/codex/config-advanced/#project-root-detection)
- PR: #8359
- PR: #10317