nix, hive-sh4re: name modules that exist in the stale harness-base refs

harness-base.nix has never existed in this tree. Four comments named it,
or a `harness-base` module, as the place to look:

- weston-vnc.nix: the agent user is declared and home-chowned by
  nix/agent-modules/user.nix
- hive-ci.nix: the sandbox-fallback reasoning lives in
  nix/agent-modules/default.nix -- which the very next comment block in
  the same file already cites correctly
- packages/default.nix: the per-bin consumer is
  nix/agent-modules/packages.nix
- hive-sh4re/src/assets.rs: HIVE_ASSETS_DIR is set by
  hive-c0re/environment.nix and agent-modules/default.nix +
  agent-service.nix, and the package is built by nix/packages/assets.nix
  -- not the equally nonexistent nix/assets.nix

assets.rs was twice declared out of scope on the sibling PR because it
names a module rather than a file. That distinction was real and
irrelevant: neither the module nor the nix/assets.nix path it points at
exists. Reading the wording is not checking the reference.

Every replacement path was verified to exist, with a deliberately bogus
path as a control.

nix/host-modules/hive-ci.nix

@@ -280,7 +280,7 @@ in
           # sandboxing always fails. Fall back to unsandboxed builds.
           # Moot once every nix invocation in the container routes
           # through the host daemon (the daemon governs sandboxing).
-          # See docs/gotchas.md and harness-base.nix.
+          # See docs/gotchas.md and nix/agent-modules/default.nix.
           nix.settings.sandbox-fallback = lib.mkForce true;
           # Degrade to a local build when a remote builder is unreachable
           # rather than failing the check. `fallback` is a client-side