[codex] simplify docs and standardize runtime layout

[Che-Zhu]

Summary

• Rename the default session runtime from local to embedded in code, Devbox bootstrap env, and README docs.
• Add runtime parsing tests for the new accepted values and explicit rejection of local.
• Move Rust sources from rust-src/ to standard Cargo layout under src/, with the CLI binary at src/bin/codex-gateway-cli.rs.
• Simplify README/README_zh as the maintained human docs, ignore generated .qoder/, and remove stale hand-maintained docs/ files.
• Clarify that /api/deployments is reserved for the Brain application rather than a general deployment product surface.
• Remove the lab UI style gallery route and tracked gallery file from the Rust gateway.
• Tighten the HTTP integration wait condition so it observes completed turn state, not only assistant text.

Validation

• cargo fmt --check
• cargo test
• cargo metadata --no-deps --format-version 1

Notes

Generated .qoder/ documentation is intentionally ignored and not modified. Existing untracked Brain/Devbox lab/ draft files were left out of this PR.