docs(explore-notes): document the ConPTY env race and open-path dedupe fixes

docs/explore-notes/conpty-sessions.md

@@ -1,7 +1,7 @@
 # ConPTY interactive sessions & launch specs
 
 > **Explore-note — verify before trusting.** Distilled map of a subsystem, not authoritative.
-> Last verified against commit `aac84e4` (2026-08-06).
+> Last verified against commit `176ba78` (2026-08-06).
 > Drift check: `git log --oneline bdee731..HEAD -- src/ClaudeDo.Worker/Planning src/ClaudeDo.Worker/Hub src/ClaudeDo.Worker/Runner/ClaudeArgsBuilder.cs src/ClaudeDo.Ui/ViewModels/MissionControlViewModel.cs src/ClaudeDo.Ui/Views/InteractiveTerminalView.axaml`
 > Stable structure only (no line numbers). See docs/explore-notes/README.md.
 
@@ -116,6 +116,35 @@ add/remove/column change; focus-mode tab switches re-present content). Two-part
 (`InteractiveTerminalViewModel.IsStarting`) and in place of the refine button while
 `TaskRowViewModel.IsRefining`.
 
+### ⚠️ Gotcha: env-var launch race across sessions
+
+`PtyTerminalSession.StartAsync` applies `TerminalLaunchDescriptor.Env` via
+`Environment.SetEnvironmentVariable` onto the **whole UI process** (Porta.Pty has no per-launch
+env seam — it always inherits the calling process's environment), then calls
+`TerminalControl.LaunchProcess()`. Two sessions starting back-to-back (e.g. planning sessions for
+two different tasks) could interleave: task B's `SetEnvironmentVariable` calls could land between
+task A's env-set and its `LaunchProcess()` fork, so task A's `claude` process inherits B's env
+(e.g. `CLAUDEDO_PLANNING_TOKEN`) and fails its own MCP auth. Fixed by serializing the
+set-env-then-launch critical section behind a process-wide `static SemaphoreSlim(1,1)` in
+`PtyTerminalSession`. Env leakage onto the whole process *after* a launch has forked remains a
+documented limitation — only the fork-time race is closed.
+
+### ⚠️ Gotcha: open-path dedupe races
+
+`MissionControlViewModel.OpenConPtySessionAsync` / `OpenPlanningConPtySessionAsync` dedupe by
+`TaskId` against `ConPtySessions`, but the check ran before an **awaited** DB title lookup and
+only `AddConPtyPane` registers the pane — two rapid invocations for the same task (e.g. a
+double-click) could both pass the dedupe check before either pane existed, opening two panes.
+`OpenMergeHelperConPtySessionAsync` was worse: it awaits `CreateMergeHelperTaskAsync` (which mints
+a brand-new task id every call) *before* any `TaskId` dedupe is even possible, so a double-trigger
+always minted two host tasks in the DB.
+
+Fixed with synchronous, pre-await claims: `_pendingTaskOpens` (shared by the two `TaskId`-keyed
+open paths) and `_pendingMergeHelperLists` (keyed by `listId`, guarding the whole method since
+there's no `TaskId` yet to dedupe on) are `HashSet<string>` fields checked-and-added at method
+entry, before any `await`, and released in a `finally`. A second overlapping call for the same key
+bails out immediately instead of racing past the collection-based dedupe.
+
 ## Focus / key handling
 
 `InteractiveTerminalView` lives in `MissionControlWindow`, so the `FocusClearing` Escape handler