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  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 ).

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/agent.example.toml

@@ -60,6 +60,24 @@ password = "changeme"
 # Dune instances do not use SteamCMD (Docker images); the steam_update func
 # will return a clear error if invoked on a dune instance.
 
+# --- Dune: Awakening (container-managed) ---------------------------------
+# Dune runs as a docker-compose stack, not a spawned process — leave
+# `executable` unset and add an [instance.docker_compose] block. The agent
+# drives `docker compose up -d / stop / restart` for start/stop/restart, and
+# `steam_update` is rejected (Dune ships as Docker images).
+#
+# [[instance]]
+# id = "dune-main"
+# game = "dune"
+# root = "/opt/dune"            # directory the compose commands run in
+# label = "Arrakis (battlegroup)"
+#
+# [instance.docker_compose]
+# file = "docker-compose.yml"   # -f; relative to root. Omit to use compose's discovery
+# project = "dune-main"         # -p; defaults to the instance id
+# service = "gameserver"        # limit lifecycle to one service; omit for the whole stack
+# command = ["docker", "compose"]   # default; use ["docker-compose"] for the legacy binary
+
 [prober]
 interval_seconds = 300