docs(nh3-dev): add Mac Pro migration plan; retire vestigial volva/heid daemons

- migration-plan.md: workstation-only migration of nh3-dev's dev env to
  the M2 Ultra Mac Pro Rack (on-subnet, sidecars stay on Linux).
- nh3-dev README: drop volva from live sidecars + add retired note.
  volva.service/heid.service removed by infra-ops at heid's request
  (re-architected daemon->CC-orchestrator; volva.service was crash-looping).

servers/nh3-dev/README.md

@@ -20,8 +20,6 @@ local Bash already executes here — no SSH-to-self needed for non-privileged wo
   behind Caddy; OSC52 clipboard shim). User systemd services under `~/.config`.
 - **mead-hall** — Bifrost tool-provider sidecar (`:5173`), CI-deployed from
   `vh/mead-hall`.
-- **volva** — Codex peer agent on the althing bus (single-turn oracle, systemd
-  daemon).
 - **bloom_music dev** — `~/development/bloom_music`; its `web/` test harness uses
   Playwright headless Chromium for OSMD browser-geometry assertions.
 
@@ -47,3 +45,8 @@ Available to **every user/project** on this box — no per-home `playwright inst
   (`scripts/refresh-server-info.sh nh3-dev`).
 - Not in the colo Docker-stack topology — no `/opt/docker/compose` deploy target;
   workloads are systemd services + dev checkouts.
+- **Retired (2026-06-08):** `volva.service` + `heid.service` user systemd units
+  removed. Heid/Volva were re-architected from Python systemd daemons (`volva run`
+  / `heid run` pollers) into Claude Code session orchestrators (heid commit
+  `12aa5a9`); the `~/development/volva` dir + venvs are gone. `volva.service` had
+  been crash-looping `203/EXEC`. Cleanup done by infra-ops at heid's request.