Add Serena configuration and project memories

(Set up Serena IDE integration with a Nix language server and create a
memory graph documenting package conventions, core structure, tech
stack, and task completion workflows.)

.serena/memories/tech_stack.md

@@ -0,0 +1,11 @@
+# nix-overlay — tech stack
+
+- Language: Nix. Build system: Nix flake via `numtide/blueprint` (with `flake-parts`, `systems` for multi-system).
+- Formatting: `treefmt-nix` → `nixfmt` on `*.nix` (`flake.lock` excluded in `treefmt.toml`).
+- Multi-input nixpkgs setup (flake.nix) — pins matter, do not "unify" them:
+  - `nixpkgs` = nixos-unstable (default; blueprint follows it).
+  - `nixpkgs-latest` = nixos-unstable — needed by packages requiring newer toolchains: rustc >= 1.95 (mcp-gateway >= 3.4.0), go >= 1.26.3 (kubernetes-mcp-server >= 0.0.66).
+  - `nixpkgs-torch211` = pinned rev `549bd84d6279f9852cae6225e372cc67fb91a4c1` — freetoken pins torch>=2.11,<2.12 / triton==3.6.0; nixos-unstable ships torch 2.13/triton 3.7, so freetoken builds via `inputs.nixpkgs-torch211.legacyPackages.${system}.callPackage` (see `packages/freetoken/default.nix`).
+  - `bun2nix` pinned to branch `staging-2.1.0` until catalog support (bun2nix#86) lands on default branch (TODO #4001).
+- Package kinds in `packages/`: mostly Rust (rustPlatform.buildRustPackage), Go, Python/torch, Node/bun; each self-contained in its own dir.
+- `flake.lock` managed only via `nix flake update` — never hand-edited.