Honor project_root_markers when discovering AGENTS.md

[etraut-openai]

Fixes #12128

The docs indicates that project_root_markers are used to discover the project root for local config as well as AGENTS.md. It looks like it was never wired up to support the latter.

Summary

• resolve project docs by walking to the configured project_root_markers (or defaults) instead of assuming the Git root, while honoring CLI overrides and handling malformed configs
• fall back to the project’s canonical path chain and add a test that makes sure custom markers upstream of .git are respected

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

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