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/conventions.md

@@ -0,0 +1,23 @@
+# nix-overlay — package conventions
+
+## Directory pattern (every package)
+`packages/<name>/default.nix` (wrapper, receives blueprint args) + `packages/<name>/package.nix` (the derivation, `pkgs.callPackage`-style argsets).
+
+Wrapper variants:
+- Plain: `{ pkgs, ... }: pkgs.callPackage ./package.nix { }`
+- Needs blueprint extras: pass `perSystem`, `inputs`, etc. through explicitly.
+- Alternate nixpkgs (freetoken): wrapper ignores `pkgs` for the derivation and calls `inputs.nixpkgs-torch211.legacyPackages.${pkgs.stdenv.hostPlatform.system}.callPackage ./package.nix { }`.
+
+## Derivedivation requirements
+- Use `stdenv.mkDerivation rec { pname; version; }` or `rustPlatform.buildRustPackage` (Rust: `cargoHash`; Go/Python equivalents as needed).
+- `meta.description` required (feeds the default meta-package listing).
+- `meta.mainProgram` set for CLI-providing packages.
+- Organizational attrs via `passthru`: `category` (e.g. "AI Coding Agents"), `hideFromDocs = true` to exclude from docs listing.
+- No `with pkgs;` at top level of Nix files (scope issues).
+- No hardcoded system paths/system assumptions.
+
+## Adding a package (sequence)
+mkdir → package.nix → default.nix wrapper → `git add packages/<name>/` (required for blueprint discovery) → `nix build .#<name>` → update README Available Packages table → commit (conventional-commit format).
+
+## Naming
+- Package dir name = flake output attr name (kebab-case).