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.

README.md

@@ -61,8 +61,8 @@ Minimal `flake.nix` for a host that runs hive-c0re:
       modules = [
         hyperhive.nixosModules.default  # hive-c0re + hive-forge in one import
         ({ ... }: {
-          hyperhive.enable = true;
-          # hyperhive.c0re.operatorPronouns = "they/them";  # default: "she/her"
+          services.hyperhive.enable = true;
+          # services.hyperhive.c0re.operatorPronouns = "they/them";  # default: "she/her"
 
           # ... rest of your host config
           system.stateVersion = "25.11";
@@ -78,7 +78,7 @@ manager container, and auto-rebuilds any container whose hyperhive
 rev goes stale. `claude-code` is unfree — hyperhive scopes the
 whitelist to itself, nothing for the operator to set.
 
-Optional: set `hyperhive.c0re.preBuildAgentTemplates = true;`
+Optional: set `services.hyperhive.c0re.preBuildAgentTemplates = true;`
 to pre-fetch the per-container system closures into your host's
 /nix/store as part of `nixos-rebuild`. First-agent-spawn then
 completes in seconds instead of minutes (no nixpkgs/claude-code
@@ -88,19 +88,20 @@ system closure. Off by default (the toplevels are pinned to
 Alternatively warm the store manually:
 `nix build git+https://forge.darkest.space/hyperhive/hyperhive#agent-base-toplevel`.
 
-Optional: set `hyperhive.domain = "example.com";` to define the
+Optional: set `services.hyperhive.domain = "example.com";` to define the
 canonical hostname for hyperhive subsystems that need a stable public
 name. No default — subsystems that require it (currently:
-`hyperhive.matrix`) assert non-null at eval time with a clear error
-message if it is missing.
+`services.hyperhive.matrix`) assert non-null at eval time with a clear
+error message if it is missing.
 
-Optional: set `hyperhive.matrix.enable = true;` to spin up a private
-[matrix-tuwunel](https://github.com/matrix-construct/tuwunel) homeserver
-in a nixos-container. Requires either `hyperhive.domain` or
-`hyperhive.matrix.serverName` to be set (eval fails with a clear error
-if both are absent). The `server_name` (embedded irrevocably in every
-user and room ID) defaults to `matrix.<domain>`; override with
-`hyperhive.matrix.serverName = "chat.example.com";` if needed. State
+Optional: set `services.hyperhive.matrix.enable = true;` to spin up a
+private [matrix-tuwunel](https://github.com/matrix-construct/tuwunel)
+homeserver in a nixos-container. Requires either
+`services.hyperhive.domain` or `services.hyperhive.matrix.serverName`
+to be set (eval fails with a clear error if both are absent). The
+`server_name` (embedded irrevocably in every user and room ID)
+defaults to `matrix.<domain>`; override with
+`services.hyperhive.matrix.serverName = "chat.example.com";` if needed. State
 lives at `/var/lib/nixos-containers/hive-matrix/`. Federation is
 enabled with an empty `trusted_servers` list; e2ee is deferred to
 a follow-up (#551).