Nested per-repo config.toml layering (ASP.NET web.config - style)

Feature request: nested per-repo config layering (web.config-style) for Codex CLI.

Allow .codex/config.toml in a repo (and optionally nested root->CWD) to override the global config so each repo can define its own MCP servers (e.g., one SQL DB per repo) without env hacks. Local .codex/config.toml should be automatically read and processed. Please decide exact scope/merging; CLI -c continues to override.