yarr needs rustc 1.97.1 but the main nixpkgs input (pinned by flake.lock)
has rustc 1.95.0. Bumping the main input breaks aioimaplib (email server
dep), so add a separate rust-nixpkgs input pinned to a newer rev.
yarr is excluded from the overlay (which uses the main nixpkgs) — build
it via 'nix build .#yarr' on the flake directly.
Replaces the Node.js-based mcp-arr-server and jellyfin-mcp backends
in the gateway with a single Rust binary that covers both *arr and
Jellyfin. Rust binaries work through the gateway's stdio spawn
(where Node.js ones fail with 401).
hermes-agent