wip: Campaign LA LA2 probe mode + idle policy — INCOMPLETE, stopped mid-task

Agent was stopped for token budget. Landed here: probe flag through
LiveSessionConnectOptions + the StartCore short-circuit, the mode field
with JsonRequired-to-semantic-validation move, host exit-code mapping,
and 34 passing tests including 3 new probe tests (agent last reported
green before the stop). NOT DONE: the idle-policy unit tests (next
step), full-suite verification, and the WSL run.

Build/test state UNVERIFIED at this commit. Next session: finish idle
policy tests, run Runtime+Headless Release suites Windows and WSL, then
dispatch the Opus dual-lens review.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.Runtime/GameRuntimeCommands.cs

@@ -27,6 +27,14 @@ public enum RuntimeSessionStartStatus
     Failed,
     Inactive,
     StaleGeneration,
+    /// <summary>
+    /// Campaign LA slice LA2: mirrors
+    /// <see cref="Session.LiveSessionStartStatus.ProbeComplete"/> — a probe
+    /// session connected, reported its roster, and gracefully disconnected
+    /// before EnterWorld. A SUCCESS outcome for the headless process host's
+    /// exit-code mapping, not a failure.
+    /// </summary>
+    ProbeComplete,
 }
 
 public readonly record struct RuntimeSessionStartResult(