feat: add support for project_root_markers in config.toml

[bolinfest]

• allow configuring project_root_markers in config.toml (user/system/MDM) to control project discovery beyond .git
• honor the markers after merging pre-project layers; default to [".git"] when unset and skip ancestor walk when set to an empty array
• document the option and add coverage for alternate markers in config loader tests

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• -> feat: add support for project_root_markers in config.toml #8359
• feat: support in-repo .codex/config.toml entries as sources of config info #8354