chore(memory): note nix shell for ad-hoc nixpkgs tooling

Record that any nixpkgs tool can be run on the fly via nix run / nix
shell, so a missing command during development is not a dead end. Takes
effect after a home-manager rebuild.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -54,7 +54,7 @@ jobs:
 
       - name: Install Nix
         if: steps.detect.outputs.run == 'true'
-        uses: cachix/install-nix-action@a49548c11d9846ad46ecc0115273879b045f001c # v31
+        uses: cachix/install-nix-action@8aa03977d8d733052d78f4e008a241fd1dbf36b3 # v31
         with:
           extra_nix_config: |
             experimental-features = nix-command flakes

CLAUDE.md

@@ -0,0 +1,63 @@
+# Working on this flake
+
+Project notes for changes to this repository. Persona and memory rules live in
+the user-global config; this file is about the flake's checks and conventions.
+
+## Before you commit: run the formatter
+
+Formatting and linting are driven by the flake. CI (`.gitea/workflows/ci.yaml`)
+runs `nix flake check`, which fails the build if any file is unformatted or trips
+a lint. From the repo root:
+
+- `nix fmt` — format the whole tree (writes changes).
+- `nix flake check` — run every check read-only (what CI runs).
+- `nix develop` — dev shell; its `shellHook` installs the git pre-commit hooks so
+  the same gates run on `git commit`.
+
+Never commit with `--no-verify`. A bypassed commit ships unformatted content and
+turns CI red on the next push to `main` (see "Docs are checked too").
+
+## What gets checked
+
+Defined in `flake.nix` (the `treefmt`, `pre-commit`, and `checks` blocks) and
+`statix.toml`:
+
+| Check        | Tool                              | Covers                                                  |
+| ------------ | --------------------------------- | ------------------------------------------------------- |
+| `formatting` | treefmt → `nixfmt`                | all `*.nix`                                             |
+| `formatting` | treefmt → `shfmt`                 | shell scripts                                           |
+| `formatting` | treefmt → `prettier`              | **Markdown, YAML, JSON** (incl. `README.md`, this file) |
+| `deadnix`    | deadnix                           | dead Nix bindings (`--no-lambda-pattern-names`)         |
+| `statix`     | statix                            | Nix antipatterns (config in `statix.toml`)              |
+| pre-commit   | nixfmt-rfc-style, deadnix, statix | the same gates, run on commit                           |
+
+Excluded from formatting: `*/hardware-configuration.nix` (generated by
+`nixos-generate-config`) and `flake.lock`. Editor defaults (indent, EOL, final
+newline) are in `.editorconfig`; note Markdown keeps trailing whitespace, which
+encodes hard line breaks.
+
+## Docs are checked too — the common trap
+
+prettier formats `*.md`, so **documentation edits must be run through `nix fmt`**
+exactly like code. prettier re-aligns Markdown tables in particular; hand-editing
+a table almost always leaves it non-conformant and fails the `formatting` check.
+
+Beware a false green: the CI `detect` step skips the heavy checks on a pull
+request that touches **no** `.nix`, `flake.lock`, or the workflow file — so a
+docs-only PR reports success without ever running prettier. The failure then
+surfaces on the push-to-`main` run (which always runs the full check) or on the
+next unrelated PR that does touch Nix. Run `nix flake check` locally before
+merging a docs change, regardless of what the PR check shows.
+
+## Host evaluation
+
+CI also evaluates every `nixosConfigurations` / `darwinConfigurations` host's
+toplevel (eval only, no build) on an x86_64 runner, so eval errors fail cheaply.
+Reproduce locally:
+
+```sh
+nix eval --raw ".#nixosConfigurations.<host>.config.system.build.toplevel.drvPath"
+```
+
+Host lists are discovered from the flake, so adding or removing a host needs no
+change to the workflow.

home/claude/memory/MEMORY.md

@@ -9,3 +9,4 @@
 - [Review and comments workflow](workflow_review_and_comments.md) — show PR body and non-trivial Jira comments before posting; terse IaC code comments; PR body content rules
 - [Sandbox prompts](feedback_sandbox_prompts.md) — don't prompt for sandbox-disable or routine read-only shell ops; broaden permissions instead
 - [Dev clusters disposable](dev_clusters_disposable.md) — Lyra's dev clusters are recreatable; mutate/break freely, no confirmation needed
+- [Nix shell tooling](nix_shell_tooling.md) — any nixpkgs tool runs ad hoc via `nix run`/`nix shell nixpkgs#<pkg>`; a missing command is never a dead end

home/claude/memory/nix_shell_tooling.md

@@ -0,0 +1,23 @@
+---
+name: nix-shell-tooling
+description: "Any nixpkgs tool can be run ad hoc via nix run / nix shell — a missing command is never a dead end during development"
+metadata:
+  node_type: memory
+  type: feedback
+  originSessionId: dfb56b58-518b-4daf-b531-7119bb4a9534
+---
+
+Any tool in nixpkgs can be run without installing it into the environment. If a
+command is missing during development, pull it from nixpkgs on the fly instead
+of working around its absence or reporting the tool as unavailable.
+
+**Why:** Lyra runs NixOS; the ambient PATH is deliberately minimal, but the full
+nixpkgs set is always one command away. "command not found" is not a blocker.
+
+**How to apply:**
+
+- One-off run: `nix run nixpkgs#<pkg> -- <args>` (e.g. `nix run nixpkgs#jq -- .`).
+- Tools on PATH for a session: `nix shell nixpkgs#<pkg> [nixpkgs#<pkg2> ...]`,
+  then run commands normally.
+- Legacy form also works: `nix-shell -p <pkg> --run '<cmd>'`.
+- Prefer this over hand-rolling a substitute for a tool that exists in nixpkgs.