Add standalone flakes to examples (#1539)

## Summary

- add standalone `flake.nix` files for every Nix-backed example
- point each example at `github:indexable-inc/index`
- pin each example to `github:NixOS/nixpkgs/nixos-unstable` and make
Index follow that nixpkgs
- use `ix.nix` as the example entrypoint, not `default.nix` or
`example.nix`
- expose fleet examples under `ix.fleets.default` for the `ix up`
toplevel contract, while preserving `nixosConfigurations`
- update aggregate example discovery to read `examples/**/ix.nix`

Fixes #1537

## Tests

- `nix run nixpkgs#nixfmt -- examples/**/*.nix lib/discovery.nix`
- coverage script: no `examples/**/default.nix` or
`examples/**/example.nix`; every example flake has an `ix.nix`; every
example flake has the Index input, nixos-unstable input, and nixpkgs
follow
- eval script: 20 fleet examples evaluated
`.#ix.fleets.default.planValue.order`; 2 non-Nix OCI examples evaluated
`.#ix.images.default.name`; `nomad-secret-refs` evaluated
`.#checks.x86_64-linux.default.name`; `nixos-switch-multi` evaluated
`.#ix.nixosConfigurations.default.web.config.system.build.toplevel.drvPath`
- aggregate eval: `.#packages.aarch64-darwin.dev-fleet-up.name`,
`.#packages.aarch64-darwin.nginx-lifecycle-up.name`,
`.#packages.aarch64-darwin.ray-cluster-up.name`

(sent by an AI agent via Codex)

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> [!NOTE]
> ### Add standalone flake entrypoints to all examples and rename
`default.nix` to `ix.nix`
> - Each example directory gains a `flake.nix` that imports `./ix.nix`
and exposes standard outputs (`ix.fleets.default`,
`nixosConfigurations`, etc.), making every example runnable directly
with `nix` or `ix up`.
> - All existing `default.nix` entrypoints are renamed to `ix.nix`; no
logic changes in these files.
> - The `dev-fleet` example is restructured: `ix.nix` becomes a `mkDev`
entrypoint and the NixOS module content moves to a new
[`dev.nix`](https://github.com/indexable-inc/index/pull/1539/files#diff-fb2fbe8cb5cba7d162ca3247836edcc33a80e0ffcb38297fb76bdebd868cf2fd).
> -
[`lib/discovery.nix`](https://github.com/indexable-inc/index/pull/1539/files#diff-99ac7778993e1b9c08033c2057dd9cb20220148285ea3f1d1f4e528b82f4a21e)
is updated to target `ix.nix`, filter examples that accept an `index`
argument, and require `nodes` and `planValue` attributes on the result.
> -
[`tests/default.nix`](https://github.com/indexable-inc/index/pull/1539/files#diff-1cc580de297308d93d82f7b72446ae4b98832a8aae3378e9e134102519a0e33a)
is updated to reference `ix.nix` paths and asserts that the `dev-fleet`
example exposes both `ix.nix` and `dev.nix`.
>
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Author: andrewgazelka

examples/_non-nix-oci/debian/flake.nix

@@ -0,0 +1,21 @@
+{
+  description = "ix example: _non-nix-oci-debian";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    index = {
+      url = "github:indexable-inc/index";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    { index, ... }:
+    let
+      image = import ./ix.nix { inherit index; };
+    in
+    {
+      ix.images.default = image;
+      packages.x86_64-linux.default = image;
+    };
+}

examples/_non-nix-oci/debian/default.nix examples/_non-nix-oci/debian/ix.nixexamples/_non-nix-oci/debian/default.nix renamed to examples/_non-nix-oci/debian/ix.nix

@@ -0,0 +1,21 @@
+{
+  description = "ix example: _non-nix-oci-ubuntu";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    index = {
+      url = "github:indexable-inc/index";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    { index, ... }:
+    let
+      image = import ./ix.nix { inherit index; };
+    in
+    {
+      ix.images.default = image;
+      packages.x86_64-linux.default = image;
+    };
+}

examples/_non-nix-oci/ubuntu/flake.nix

@@ -27,7 +27,7 @@ ix shell client -- curl -fsS http://api:8080/
 
 ## Shape
 
-- [`default.nix`](default.nix) declares the fleet; note `api` has no
+- [`ix.nix`](ix.nix) declares the fleet; note `api` has no
   fleet-level `groups` entry.
 - [`api.nix`](api.nix) carries the group in the image
   (`ix.networking.groups`) and exposes port 8080 to group members.

examples/_non-nix-oci/ubuntu/default.nix examples/_non-nix-oci/ubuntu/ix.nixexamples/_non-nix-oci/ubuntu/default.nix renamed to examples/_non-nix-oci/ubuntu/ix.nix

@@ -0,0 +1,21 @@
+{
+  description = "ix example: declared-groups";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    index = {
+      url = "github:indexable-inc/index";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    { index, ... }:
+    let
+      fleet = import ./ix.nix { inherit index; };
+    in
+    {
+      ix.fleets.default = fleet;
+      nixosConfigurations = fleet.nixosConfigurations;
+    };
+}

examples/declared-groups/README.md

@@ -1,34 +1,33 @@
 # ix fleet
 
-A forkable ix environment (RFC 0007). One [`ix.nix`](ix.nix), an ordinary
-NixOS module, is the source of truth for the per-VM environment, the fleet
-topology, and an opt-in shared SMB volume that gives the whole fleet one Claude
-(and ix) login.
+A forkable ix environment (RFC 0007). [`ix.nix`](ix.nix) is the runnable ix
+entrypoint, and [`dev.nix`](dev.nix) is the ordinary NixOS module for the
+per-VM environment, fleet topology, and opt-in shared SMB volume that gives the
+whole fleet one Claude (and ix) login.
 
 ## Run
 
 ```sh
-nix run .#dev-fleet-up
+ix up
 ```
 
-Run that from this repo root. A copied consumer flake can use `ix up` after it
-exposes the `nixosConfigurations` returned by `index.lib.mkDev`; `ix.nix` is the
-editable module, not the CLI entrypoint.
+Run that from a copied example flake. In this repo root, the aggregate example
+wrapper still exposes `nix run .#dev-fleet-up`; the standalone example shape is
+what `ix up` consumes.
 
 This example declares a multi-node `ix.dev.fleet`. Omit that block and the same
-`ix.nix` is a **single VM named `dev`** that `index.lib.mkDev` builds and
-creates through `nix run .#up`; `default.nix` shows the composition. The fleet
-below is the scale-up.
+`dev.nix` is a **single VM named `dev`** that `index.lib.mkDev` builds and
+creates through `ix up`; `ix.nix` shows the composition. The fleet below is the
+scale-up.
 
 ## Shape
 
-- [`ix.nix`](ix.nix) is the module a user edits after `ix init`. Top-level
+- [`dev.nix`](dev.nix) is the module a user edits after `ix init`. Top-level
   NixOS config (`environment.systemPackages`, `programs.git.enable`) is the
   environment; `ix.dev.fleet` is the topology; `ix.dev.shared` turns on the
   identity volume. Claude Code and Codex are installed by default.
-- [`default.nix`](default.nix) hands the module to `index.lib.mkDev`, passing
-  `src = ./.` (the flake source a copied repo wires as `self`). In this repo,
-  fleet discovery imports `default.nix` and exposes `.#dev-fleet-up`.
+- [`ix.nix`](ix.nix) hands the module to `index.lib.mkDev`, passing
+  `src = ./.` (the flake source a copied repo wires as `self`).
 
 `mkDev` reads `ix.dev` and desugars this into a `mkFleet` plan:
 
@@ -65,7 +64,7 @@ notes; this example places the source.)
 ## Tradeoffs
 
 - The share is **guest-writable** by default (`ix.dev.shared.guestOk`) so
-  bring-up works without secrets plumbing, the same tradeoff
+  `ix up` works without secrets plumbing, the same tradeoff
   [`multi-client-file-sharing`](../multi-client-file-sharing) documents. It is
   only reachable on the private group, never public. A real shared-auth volume
   should set `guestOk = false`, add a Samba user, and pass `credentials=`

examples/declared-groups/flake.nix

@@ -0,0 +1,30 @@
+# A forkable ix environment (RFC 0007). This is an ordinary NixOS module: write
+# your environment at the top level, and use `ix.dev.*` for the fleet and the
+# shared volume. After `ix init` this is the one file you edit.
+{ pkgs, ... }:
+{
+  # Your environment: applied to every VM (single or fleet).
+  environment.systemPackages = [
+    pkgs.ripgrep
+    pkgs.jq
+  ];
+  programs.git.enable = true;
+
+  # Claude Code + Codex are installed by default; toggle either off here.
+  # ix.dev.agents.codex = false;
+
+  # A fleet: two interchangeable agents plus a builder that opts out of the
+  # shared volume below. Drop `ix.dev.fleet` entirely for a single VM.
+  ix.dev.fleet = {
+    agent.replicas = 2;
+    builder.dependsOn = [ "agent" ];
+  };
+
+  # One shared Claude (and ix) login for the fleet, over an SMB volume. The
+  # first `claude login` on any agent logs in every agent; `builder` opts out.
+  ix.dev.shared = {
+    enable = true;
+    ix = true; # also share ~/.n so a node can spawn more VMs (claude is shared by default)
+    excludeNodes = [ "builder" ];
+  };
+}