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.Headless/Hosting/HeadlessProcessHost.cs

@@ -200,6 +200,22 @@ internal sealed class HeadlessProcessHost : IDisposable
         foreach (HeadlessSessionHost session in _sessions)
         {
             RuntimeSessionStartResult started = session.Start();
+            // Campaign LA slice LA2: ProbeComplete is a SUCCESS variant, not
+            // a connection failure — the session already connected, reported
+            // its roster, and gracefully disconnected before EnterWorld (see
+            // LiveSessionController's probe short-circuit). Continue to the
+            // next configured session instead of returning ConnectionError,
+            // so a probe session sharing a process with play sessions never
+            // tears the others down. ProbeHeadlessBotPolicy already reports
+            // IsComplete, so the scheduler below skips this session entirely.
+            if (started.Status == RuntimeSessionStartStatus.ProbeComplete)
+            {
+                _diagnostics.Lifecycle(
+                    session.SessionId,
+                    "probed",
+                    session.Runtime);
+                continue;
+            }
             if (started.Status != RuntimeSessionStartStatus.Connected)
             {
                 if (started.Error is { } error)