fix(nix): update flake for newer Rust toolchain requirements (#10302)

## 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

- [x] `nix develop` enters shell successfully
- [x] `nix develop -c rustc --version` shows 1.93.0
- [x] `nix develop -c cargo build` completes successfully

flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1758427187,
-        "narHash": "sha256-pHpxZ/IyCwoTQPtFIAG2QaxuSm8jWzrzBGjwQZIttJc=",
+        "lastModified": 1769461804,
+        "narHash": "sha256-msG8SU5WsBUfVVa/9RPLaymvi5bI8edTavbIq3vRlhI=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "554be6495561ff07b6c724047bdd7e0716aa7b46",
+        "rev": "bfc1b8a4574108ceef22f02bafcf6611380c100d",
         "type": "github"
       },
       "original": {
@@ -18,7 +18,28 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1769828398,
+        "narHash": "sha256-zmnvRUm15QrlKH0V1BZoiT3U+Q+tr+P5Osi8qgtL9fY=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "a1d32c90c8a4ea43e9586b7e5894c179d5747425",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
       }
     }
   },