nix: pivot to services.hyperhive.* per mara directive (#612)

Per [mara on PR #615 comment 7349](http://localhost:3000/hyperhive/hyperhive/pulls/615#issuecomment-7349):
> follow nix conventions, services.hyperhive it is. the earlier we
> change this, the less breakage.

Renames the entire host-side option tree under `services.hyperhive.*`:

- `services.hive-c0re.*` → `services.hyperhive.c0re.*`
- `hyperhive.enable` → `services.hyperhive.enable`
- `hyperhive.domain` → `services.hyperhive.domain`
- `hyperhive.forge.*` → `services.hyperhive.forge.*`
- `hyperhive.matrix.*` → `services.hyperhive.matrix.*`

Per mara's "earlier = less breakage", the previous `services.hive-c0re.enable`
deprecation alias is dropped. Operators get a clear eval error on the
old paths pointing at the rename. Single migration moment.

Per-agent options in `nix/templates/harness-base.nix` (`hyperhive.model`,
`hyperhive.allowedRecipients`, etc.) stay at `hyperhive.*` — they're
container-level config, not services in the NixOS sense.

Verified via `nix flake check --no-build` + an end-to-end NixOS eval
exercising every renamed path.

Follow-up needed: rust source comments referencing the old NixOS
option names (`hive-c0re/src/{meta,coordinator,main,dashboard}.rs`)
should be updated in a separate pure-rust PR to keep this one
strictly nix-only.

nix/templates/harness-base.nix

@@ -36,7 +36,7 @@
       `"haiku"`, `"sonnet"`, `"opus"` (or any future identifier). Context
       window sizes are looked up at runtime from the
       `HIVE_CONTEXT_WINDOW_TOKENS_<KEY_UPPER>` env vars injected by the
-      meta flake; override sizes via `hyperhive.c0re.contextWindowTokens`
+      meta flake; override sizes via `services.hyperhive.c0re.contextWindowTokens`
       on the host.
     '';
   };
@@ -595,7 +595,7 @@
     # both the harness binary and any user-shell `cargo run` inside the
     # container resolve them from the same path.
     # HIVE_CONTEXT_WINDOW_TOKENS_* are injected by the meta flake from the
-    # host-level `hyperhive.c0re.contextWindowTokens` option — not set here.
+    # host-level `services.hyperhive.c0re.contextWindowTokens` option — not set here.
     environment.variables = {
       HIVE_DEFAULT_MODEL = config.hyperhive.model;
       HIVE_ASSETS_DIR = "${pkgs.hyperhive-assets}/share/hyperhive";