fix(headless): N3 reject --console for validate mode instead of ignoring it

validate never starts a session, so a silently-ignored --console read as
"it worked" to an operator who typo'd their command. Reject with a clear
message instead.

New test ValidateModeRejectsTheConsoleFlag was shown to fail first
(mutation: the guard absent — HeadlessCommandLine.Parse returned normally
for `validate --config bot.json --console` instead of throwing).

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

src/AcDream.Headless/Configuration/HeadlessCommandLine.cs

@@ -95,6 +95,14 @@ internal sealed record HeadlessCommandLine(
             throw new HeadlessCommandLineException(
                 "Direct credentials are valid only for run mode.");
         }
+        // N3: reject rather than silently ignore --console for validate mode
+        // — validate never starts a session, so there is nothing for the
+        // console to attach to.
+        if (console && command != "run")
+        {
+            throw new HeadlessCommandLineException(
+                "--console is valid only for run mode.");
+        }
 
         return new HeadlessCommandLine(
             command,

tests/AcDream.Headless.Tests/HeadlessConsoleTests.cs

@@ -69,6 +69,19 @@ public sealed class HeadlessConsoleTests
         Assert.False(parsed.ConsoleEnabled);
     }
 
+    /// <summary>
+    /// N3: validate mode rejects <c>--console</c> outright (rather than
+    /// silently ignoring it) — validate never starts a session, so there is
+    /// nothing for the console to attach to.
+    /// </summary>
+    [Fact]
+    public void ValidateModeRejectsTheConsoleFlag()
+    {
+        Assert.Throws<HeadlessCommandLineException>(() =>
+            HeadlessCommandLine.Parse(
+                ["validate", "--config", "bot.json", "--console"]));
+    }
+
     // ── HeadlessConsoleInputReader: reader-thread/ordering ───────────────
 
     /// <summary>