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.

docs/turn-loop.md

@@ -76,7 +76,7 @@ match wins):
 
 1. `HIVE_CONTEXT_WINDOW_TOKENS_<KEY>` env var, where `KEY`
    (lowercased) is a substring of the active model name. Injected
-   by the meta flake from `hyperhive.c0re.contextWindowTokens`
+   by the meta flake from `services.hyperhive.c0re.contextWindowTokens`
    (host-level NixOS option, defaults: haiku=200k, sonnet=1M,
    opus=1M). Override these for all agents at once without a
    per-agent config change.
@@ -178,7 +178,7 @@ socket at `/run/hive/` once at startup:
   #519); everything else is shared. Then `{label}` and
   `{operator_pronouns}` get substituted in the assembled output.
   Pronouns come from `HIVE_OPERATOR_PRONOUNS` env (set by the meta
-  flake from `hyperhive.c0re.operatorPronouns`, default
+  flake from `services.hyperhive.c0re.operatorPronouns`, default
   `she/her`). Passed via `--system-prompt-file`.
 
 The shared per-turn plumbing lives in `hive_ag3nt::turn::{write_mcp_config,