docs: resolve review major items in Herdr main-vertical spec

docs/superpowers/specs/2026-07-07-herdr-main-vertical-design.md

@@ -52,11 +52,11 @@ spawnPane(sessionId, description, serverUrl, directory):
        direction = paneDirection  // 'right' for main-vertical
        wasFirstChild = true
 
-  3. If we did not already split in step 2: split(target, direction)
-  4. Parse pane_id from output
-  5. If wasFirstChild AND layout === 'main-vertical':
+   3. If wasFirstChild: split(target, direction)  // 1st child or fallback path
+   4. Parse pane_id from output
+   5. If wasFirstChild AND layout === 'main-vertical':
        agentAreaPaneId = newPaneId
-  6. Rename pane, run opencode attach (existing)
+   6. Rename pane, run opencode attach (existing)
 ```
 
 Key properties:
@@ -66,13 +66,30 @@ Key properties:
 - **Explicit `wasFirstChild` flag** avoids fragile implicit detection
 - **Gated on `main-vertical` only** — other layouts unchanged
 
+### Review resolutions (from code review)
+
+**Major 1 — Stale vs. other split failures:**
+The agent-area split failure triggers a parent fallback. On a transient herdr error (not stale pane), the parent split would also likely fail → returns `{success: false}` (existing error path). No duplicate agent area is created because the fallback only *succeeds* when the agent area was genuinely stale (parent split works). Herdr has no reliable `pane list` API to distinguish failure causes, so we accept this limitation and document it. No extra error classification added (YAGNI).
+
+**Major 2 — Focus behavior:**
+`--no-focus` keeps focus on the *target* pane of the split. For the 1st child (split parent → right), focus stays on parent. For 2nd+ (split agent area → down), focus stays on the agent area pane. This is acceptable: the user monitors agents in the right column while the parent retains priority visually. Documented as intended tradeoff; no focus manipulation added (out of scope per issue acceptance criteria).
+
+**Major 3 — closePane insertion point:**
+The agent-area nulling is placed *after* the existing guard `if (!paneId || paneId === 'unknown') return true;` so it never fires for sentinel values. Exact placement: line 156 (after guard), before `getBinary()`.
+
 ### `closePane` Change
 
+Placement: after the existing `if (!paneId || paneId === 'unknown') return true;` guard (line 156), before `getBinary()`.
+
 ```typescript
 async closePane(paneId: string): Promise<boolean> {
+  if (!paneId || paneId === 'unknown') return true;
+
   if (paneId === this.agentAreaPaneId) {
     this.agentAreaPaneId = null;  // next spawn re-creates from parent
   }
+
+  const herdr = await this.getBinary();
   // ... existing logic unchanged
 }
 ```