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/agent-modules/weston-vnc.nix

@@ -6,7 +6,7 @@
 }:
 let
   # GUI processes run as the agent's own non-root user — the same user
-  # hive-agent runs as (declared + home-chowned by harness-base.nix) — so
+  # hive-agent runs as (declared + home-chowned by ./user.nix) — so
   # weston, the wayland client, and the agent share one user session.
   # `hyperhive.user.name` is set per-agent by the meta-flake renderer.
   userName = config.hyperhive.user.name;