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/Configuration/HeadlessConfiguration.cs

@@ -46,11 +46,33 @@ internal sealed record HeadlessSessionDescriptor
     [JsonRequired]
     public string Account { get; init; } = string.Empty;
 
-    [JsonRequired]
-    public HeadlessCharacterSelector Character { get; init; } = new();
+    /// <summary>
+    /// Campaign LA slice LA2: ABSENT (<see langword="null"/>) for normal play
+    /// sessions; <see cref="HeadlessSessionMode.Probe"/> for the LA2 probe
+    /// (connect → characterList → graceful disconnect, never EnterWorld) —
+    /// the pinned launch-contract schema's <c>mode</c> field
+    /// (<c>docs/plans/2026-08-14-launcher-campaign.md</c> LA1/LA2).
+    /// <see cref="Character"/>/<see cref="Policy"/> requiredness depends on
+    /// this value, which is why their requiredness lives in
+    /// <see cref="HeadlessConfigurationLoader"/>'s semantic validation rather
+    /// than a <c>[JsonRequired]</c> attribute — that attribute fires during
+    /// deserialization, before <see cref="Mode"/> can be inspected at all.
+    /// </summary>
+    public HeadlessSessionMode? Mode { get; init; }
 
-    [JsonRequired]
-    public HeadlessBotPolicyDescriptor Policy { get; init; } = new();
+    /// <summary>
+    /// Required for play sessions (<see cref="Mode"/> absent); MUST be
+    /// omitted for probe sessions (<see cref="HeadlessSessionMode.Probe"/>) —
+    /// the pinned contract keeps the shape unambiguous by forbidding a probe
+    /// session from also declaring a selector. Enforced by
+    /// <see cref="HeadlessConfigurationLoader.ValidateSession"/>, not
+    /// <c>[JsonRequired]</c> (see this record's own doc on <see cref="Mode"/>).
+    /// </summary>
+    public HeadlessCharacterSelector? Character { get; init; }
+
+    /// <summary>Same mode-dependent requiredness as <see cref="Character"/>:
+    /// required for play sessions, forbidden for probe sessions.</summary>
+    public HeadlessBotPolicyDescriptor? Policy { get; init; }
 
     [JsonRequired]
     public HeadlessCredentialReference Credential { get; init; } = new();
@@ -140,6 +162,19 @@ internal sealed class HeadlessBotPolicyDescriptor
     public HeadlessBotPolicyRole? Role { get; init; }
 }
 
+/// <summary>
+/// Campaign LA slice LA2: see <see cref="HeadlessSessionDescriptor.Mode"/>.
+/// The pinned launch-contract schema defines exactly two states for a
+/// session — ABSENT (mapped to <see langword="null"/>, meaning "play") or
+/// the literal string <c>"probe"</c> — so <see cref="Probe"/> is the only
+/// member; there is no explicit "play" spelling.
+/// </summary>
+[JsonConverter(typeof(JsonStringEnumConverter<HeadlessSessionMode>))]
+internal enum HeadlessSessionMode
+{
+    Probe,
+}
+
 /// <summary>See <see cref="HeadlessBotPolicyDescriptor.Role"/>.</summary>
 [JsonConverter(typeof(JsonStringEnumConverter<HeadlessBotPolicyRole>))]
 internal enum HeadlessBotPolicyRole