feat(host-agent): Phase 2 — Dune docker-compose adapter via Supervisor trait

Introduce a Supervisor trait (async-trait) so the agent manages games with
different models behind one wire contract. ProcessSupervisor (spawned process:
rust/conan/soulmask) and the new DockerComposeSupervisor (dune) both impl it;
Agent.supervisors is now HashMap<String, Arc<dyn Supervisor>> and instancecmd
dispatch is game-agnostic — start/stop/restart/status identical across games,
selected by a per-game factory in main. InstanceState moved to the shared
supervisor module.

DockerComposeSupervisor drives docker-compose up-d / stop / restart against
the instance's compose project, with -f/-p/single-service support and a
configurable compose binary. New [instance.docker_compose] config block.
First cut = lifecycle + cached state; container crash-detection + restart
adoption deferred to Phase 3b (reconcilable with a compose ps probe).

Trait choice (dyn over enum) per Commander: scales to future planes (kubectl,
AMP/podman, SSH) as new struct+impl, no central match.

56 tests green (6 new docker-compose mock-binary tests + 5 refactored process
tests), zero warnings. Live verification pending a real Dune stack.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

corrosion-host-agent/PROTOCOL.md

@@ -101,8 +101,16 @@ Payload: `{}`.
 
 Lifecycle and control for one game instance.
 
+The same `start`/`stop`/`restart`/`status` funcs work for **every** game: the
+agent picks a `Supervisor` impl per game — a spawned-process supervisor for
+Rust/Conan/Soulmask, a **docker-compose supervisor for Dune** (`docker compose
+up -d` / `stop` / `restart` against the instance's compose project, configured
+via `[instance.docker_compose]`). The wire contract is identical; only the
+management model behind it differs.
+
 Implemented funcs: `start`, `stop` (graceful with 30s budget, then force
-kill), `restart`, `status` (returns `state` + `uptime_seconds`), and
+kill — process supervisor; Dune maps stop to `docker compose stop`), `restart`,
+`status` (returns `state` + `uptime_seconds`), and
 `rcon` — `{ "func": "rcon", "command": "<console command>" }` returns
 `{ "status": "success", "output": <server response> }`. Protocol per game:
 WebRCON (WebSocket JSON) for rust, Source RCON (Valve TCP) for
@@ -118,7 +126,10 @@ streaming progress lines to `corrosion.{license}.{instance}.steam_status`
 and replying on completion.
 
 Planned funcs: `oxide_install` (rust), plus game-adapter-specific
-commands (Dune: docker lifecycle, RabbitMQ bus commands, Coriolis reset).
+commands (Dune: RabbitMQ admin-bus commands, Coriolis reset, Postgres admin
+surface). Dune **lifecycle** is already covered by the shared
+start/stop/restart funcs above; container crash-detection and state adoption on
+agent restart land with Phase 3b.
 
 ### `corrosion.{license_id}.{instance_id}.steam_status` (agent → backend, publish) — LIVE