feat(host-agent): add optional Appium/Runtime supervisor

Adds an opt-in dependency supervisor inside the Host Agent that probes,
spawns, and restarts the two local processes the macOS single-machine
real-device workflow depends on: the Appium server (gates Driver.connect())
and the local Runtime API (local inspection). Default-off; gated by
HOST_AGENT_DEPENDENCY_SUPERVISOR_ENABLED plus per-dependency *_SUPERVISED
flags.

Mitigates the live-incident failure mode where forgetting to start Appium
silently keeps devices offline and tasks queued forever with no error
surfaced in Host Agent logs.

Behavior (per openspec change):
- Adopt-don't-fight: probe (TCP + dependency-specific HTTP health check)
  before spawn. Healthy listener → adopted (never killed/restarted).
  Unhealthy listener → port-conflict error, skip. No listener → spawn.
- Only supervisor-spawned processes are restarted on crash, with capped
  exponential backoff (1s/2s/4s/8s, capped at 30s) and a per-process-lifetime
  attempt ceiling (HOST_AGENT_DEPENDENCY_RESTART_MAX_ATTEMPTS, default 5).
- Spawn failures (e.g. missing executable) logged distinctly from crashes.
- Graceful stop terminates only spawned children; adopted processes untouched.
- Supervisor starts before the heartbeat loop's first connect_devices() pass
  and stops alongside existing heartbeat/console teardown.

Validation: ruff check + format clean, compileall clean, openspec validate
--strict valid. Non-integration suite 503 passed / 44 deselected / 2 failed
(both failures pre-existing from unrelated 03c7c30 LLM_PROVIDER_ENC_KEY;
verified by stashing this change). macOS real-device manual verification
(task 6.4) deferred to a macOS host.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/device-host-agent/host_agent/config.py

@@ -35,6 +35,14 @@ class HostAgentConfig:
     console_session_ttl_seconds: float = 43200.0
     console_history_limit: int = 200
     ai_planner_transport: str = "direct"
+    dependency_supervisor_enabled: bool = False
+    appium_supervised: bool = False
+    appium_host: str = "127.0.0.1"
+    appium_port: int = 4723
+    runtime_supervised: bool = False
+    runtime_host: str = "127.0.0.1"
+    runtime_port: int = 8000
+    dependency_restart_max_attempts: int = 5
 
 
 def load_host_agent_config(
@@ -115,6 +123,18 @@ def load_host_agent_config(
         ai_planner_transport=_parse_ai_planner_transport(
             values.get("AI_PLANNER_TRANSPORT")
         ),
+        dependency_supervisor_enabled=_truthy(
+            values, "HOST_AGENT_DEPENDENCY_SUPERVISOR_ENABLED", False
+        ),
+        appium_supervised=_truthy(values, "HOST_AGENT_APPIUM_SUPERVISED", False),
+        appium_host=values.get("HOST_AGENT_APPIUM_HOST", "127.0.0.1").strip(),
+        appium_port=_positive_int(values, "HOST_AGENT_APPIUM_PORT", 4723),
+        runtime_supervised=_truthy(values, "HOST_AGENT_RUNTIME_SUPERVISED", False),
+        runtime_host=values.get("HOST_AGENT_RUNTIME_HOST", "127.0.0.1").strip(),
+        runtime_port=_positive_int(values, "HOST_AGENT_RUNTIME_PORT", 8000),
+        dependency_restart_max_attempts=_positive_int(
+            values, "HOST_AGENT_DEPENDENCY_RESTART_MAX_ATTEMPTS", 5
+        ),
     )
     if config.max_retry_backoff_seconds < config.retry_backoff_seconds:
         raise HostAgentConfigurationError(