fix(headless): S7 report why a multi-session process never attaches the console

A configured process with 2+ sessions and --console silently skipped
console attachment (the plan's "single-session only for the first cut"),
indistinguishable from --console simply having worked. Report it
explicitly through the same HeadlessDiagnosticWriter.Message stream
every other structured event already uses.

TwoSessionsWithConsoleFlagReportsSingleSessionOnly was shown to fail
against the silent-skip branch (mutation: the else-branch body removed)
-- the diagnostics stream carried only the ordinary lifecycle/resource
JSON lines, with no "single-session only" message anywhere.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/AcDream.Headless/Hosting/HeadlessProcessHost.cs

@@ -192,6 +192,14 @@ internal sealed class HeadlessProcessHost : IDisposable
                 };
                 console = controller;
             }
+            else if (consoleEnabled)
+            {
+                // S7 (2026-09-07 review round): a silent skip here read as
+                // "--console worked" to an operator with no way to tell
+                // otherwise — the launcher's multi-session mode is a
+                // legitimate, common configuration, so say so explicitly.
+                _diagnostics.Message("console", "single-session only");
+            }
             _console = console;
             _consoleRendererSubscription = consoleRendererSubscription;
         }

tests/AcDream.Headless.Tests/HeadlessConsoleTests.cs

@@ -564,6 +564,58 @@ public sealed class HeadlessConsoleTests
         Assert.Equal(expectDimmed, text.Contains("[2m"));
     }
 
+    /// <summary>
+    /// S7: a multi-session process with <c>--console</c> must tell the
+    /// operator why the console never attached (the launcher's multi-session
+    /// mode is a legitimate, common configuration) instead of silently
+    /// doing nothing — see <c>HeadlessDiagnosticWriter.Message</c>'s "console"
+    /// category.
+    /// </summary>
+    [Fact]
+    public void TwoSessionsWithConsoleFlagReportsSingleSessionOnly()
+    {
+        // StandardInput credentials (one line per session, consumed by
+        // HeadlessCredentialResolver BEFORE the console reader thread ever
+        // starts — see HeadlessProcessHost's own constructor comment) avoid
+        // needing an ACE-shaped Environment credential just to reach the
+        // console-wiring branch this test targets.
+        HeadlessSessionDescriptor StandardInputDescriptor(string id) =>
+            Descriptor() with
+            {
+                Id = id,
+                Credential = new HeadlessCredentialReference
+                {
+                    Provider = HeadlessCredentialProviderKind.StandardInput,
+                    Reference = "fixture",
+                },
+            };
+        var configuration = new HeadlessConfiguration
+        {
+            Version = 1,
+            Sessions =
+            [
+                StandardInputDescriptor("one"),
+                StandardInputDescriptor("two"),
+            ],
+        };
+        var operations = new FixtureSessionOperations();
+        using var diagnostics = new StringWriter();
+        using var input = new System.IO.StringReader(
+            "password-one" + Environment.NewLine
+            + "password-two" + Environment.NewLine);
+        using var host = new HeadlessProcessHost(
+            configuration,
+            HeadlessPathSet.Resolve(new HeadlessPathOverrides()),
+            input,
+            diagnostics,
+            operations,
+            new FakeTimeProvider(),
+            directCredentials: null,
+            consoleEnabled: true);
+
+        Assert.Contains("single-session only", diagnostics.ToString());
+    }
+
     private static bool WaitForEndOfInput(HeadlessConsoleController controller) =>
         controller.Reader.EndOfInput.Wait(TimeSpan.FromSeconds(5));