refactor(1b): RL5 — consolidate Nix code under nix/ (modules->nix/modules, hosts->nix/hosts)

flake.nix/flake.lock STAY at root so the build ref #cc-ci is unchanged; only flake's internal
configuration.nix path updated. Root-relative refs inside moved modules re-based ../X -> ../../X
(secrets/bridge/dashboard); configuration.nix's ../../modules imports unchanged (both dirs under nix/).
Living docs (README, architecture/install/secrets/enroll) + .drone.yml comment updated to nix/...;
append-only history logs left as-is. DECISIONS.md records RL5 + the deferred-coordinated RL6.

Verified on cc-ci: nixos-rebuild build 'path:#cc-ci' -> toplevel 8i3jcad9 (BYTE-IDENTICAL to the
pre-move build — store derivations are content-addressed on file contents, module .nix not in the
runtime closure); scripts/lint.sh -> lint: PASS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nix/modules/harness.nix

@@ -0,0 +1,20 @@
+# CI harness runtime (M4): a reproducible Python env with pytest + Playwright and the
+# Nix-provided browsers, exposed as `cc-ci-run` on the host so the Drone exec pipeline (and
+# manual dev) can run the harness with `cc-ci-run runner/run_recipe_ci.py`. Playwright on NixOS
+# needs the browsers from nixpkgs (not a downloaded copy) via PLAYWRIGHT_BROWSERS_PATH.
+{ pkgs, ... }:
+let
+  pyEnv = pkgs.python3.withPackages (ps: with ps; [ pytest playwright ]);
+  ccciRun = pkgs.writeShellApplication {
+    name = "cc-ci-run";
+    runtimeInputs = [ pyEnv pkgs.abra pkgs.docker pkgs.git pkgs.coreutils pkgs.util-linux ];
+    text = ''
+      export PLAYWRIGHT_BROWSERS_PATH=${pkgs.playwright-driver.browsers}
+      export PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD=1
+      exec ${pyEnv}/bin/python3 "$@"
+    '';
+  };
+in
+{
+  environment.systemPackages = [ ccciRun ];
+}