fix(nix): update flake for newer Rust toolchain requirements

[douglaz]

Summary

• Add rust-overlay input to provide newer Rust versions (rama crates require rustc 1.91.0+)
• Add devShells output with complete development environment
• Add missing git dependency hashes to codex-rs/default.nix

Changes

flake.nix:

• Added rust-overlay input to get newer Rust toolchains
• Updated packages output to use rust-bin.stable.latest.minimal for builds
• Added devShells output with:
  • Rust with rust-src and rust-analyzer extensions for IDE support
  • Required build dependencies: pkg-config, openssl, cmake, libclang
  • Environment variables: PKG_CONFIG_PATH, LIBCLANG_PATH

codex-rs/default.nix:

• Added missing outputHashes for git dependencies:
  • nucleo-0.5.0, nucleo-matcher-0.3.1
  • runfiles-0.1.0
  • tokio-tungstenite-0.28.0, tungstenite-0.28.0

Test Plan

• nix develop enters shell successfully
• nix develop -c rustc --version shows 1.93.0
• nix develop -c cargo build completes successfully

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[douglaz]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

@codex review

[etraut-openai]

Thanks for the contribution.

We recently updated our contribution guidelines to indicate that we're no longer accepting unsolicited code contributions. All code contributions are by invitation only. To read more about why we've taken this step, please refer to this announcement.

I'm going to exempt this PR from the "by invitation only" guidance because we just recently introduced the change to our guidance plus the contents of the PR are relatively low risk and fast to review. Please follow the new guidance for future PRs.

Does this address all of the problems mentioned in #10044?

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

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