fix(ui): serialize ConPTY env launch and close open-path dedupe races

Two sessions starting back-to-back could interleave SetEnvironmentVariable
calls before either LaunchProcess() forks, leaking one task's env (e.g.
CLAUDEDO_PLANNING_TOKEN) into another's claude process. Serialize the
set-env + LaunchProcess critical section behind a static SemaphoreSlim in
PtyTerminalSession.

OpenConPtySessionAsync/OpenPlanningConPtySessionAsync ran their TaskId
dedupe check before an awaited DB title lookup, and OpenMergeHelperConPtySessionAsync
awaited task creation before any dedupe was possible - rapid double-invocation
could open two panes or mint two host tasks. Claim the key synchronously at
method entry, before any await, and release it in a finally.

src/ClaudeDo.Ui/Services/PtyTerminalSession.cs

@@ -19,6 +19,14 @@ namespace ClaudeDo.Ui.Services;
 /// </summary>
 public sealed class PtyTerminalSession : IDisposable
 {
+    // Guards the set-env + LaunchProcess critical section below: two sessions starting
+    // back-to-back (e.g. planning sessions for two different tasks) could otherwise interleave
+    // their SetEnvironmentVariable calls before either LaunchProcess() forks, so one process
+    // inherits the other's env (e.g. CLAUDEDO_PLANNING_TOKEN, breaking that session's own MCP
+    // auth). Process-wide env leakage AFTER a launch has forked remains a documented limitation
+    // — Porta.Pty has no per-launch env seam, so the vars stay set on the whole UI process.
+    private static readonly SemaphoreSlim s_launchGate = new(1, 1);
+
     private TerminalControl? _control;
     private bool _disposed;
 
@@ -38,14 +46,22 @@ public sealed class PtyTerminalSession : IDisposable
         _control = control;
         control.ProcessExited += OnControlProcessExited;
 
-        foreach (var (key, value) in descriptor.Env)
-            Environment.SetEnvironmentVariable(key, value);
-
         control.Process = descriptor.Exe;
         control.Args = new List<string>(descriptor.Args);
         control.StartingDirectory = descriptor.Cwd;
 
-        await control.LaunchProcess();
+        await s_launchGate.WaitAsync(ct);
+        try
+        {
+            foreach (var (key, value) in descriptor.Env)
+                Environment.SetEnvironmentVariable(key, value);
+
+            await control.LaunchProcess();
+        }
+        finally
+        {
+            s_launchGate.Release();
+        }
 
         // Permanent reparent mode: TerminalView.OnDetachedFromLogicalTree kills the child
         // process unless BeginReparent() suppressed it, and Mission Control detaches pane