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.

hive-sh4re/src/assets.rs

@@ -3,9 +3,11 @@
 //! (`hive-c0re`) and the in-container harness binaries so they agree
 //! on the lookup contract.
 //!
-//! At runtime, the path is read from `$HIVE_ASSETS_DIR`. In nix
-//! builds that env var is set by the `hive-c0re` / `harness-base` modules
-//! to `${pkgs.hyperhive-assets}/share/hyperhive` (see `nix/assets.nix`).
+//! At runtime, the path is read from `$HIVE_ASSETS_DIR`. In nix builds
+//! that env var is set to `${pkgs.hyperhive-assets}/share/hyperhive` by
+//! `nix/host-modules/hive-c0re/environment.nix` (host daemon) and
+//! `nix/agent-modules/default.nix` + `agent-service.nix` (containers);
+//! the package itself is built by `nix/packages/assets.nix`.
 //! For `cargo run` outside nix, set it yourself:
 //!
 //! ```sh