feat(orchestrator): create_skill action + wire per-project watchers (§14.3)

- domain: OrchestratorCommand::CreateSkill with optional scope field
  (defaults to Project, case-insensitive, UnknownScope on bad value)
- application: dispatch CreateSkill through the CreateSkill use case
- app-tauri: build OrchestratorService and start/stop per-project request
  watchers on open/create/close_project (ensure/stop_orchestrator_watch)
- tests: domain validation, service dispatch, infra watcher e2e,
  app-tauri wiring lifecycle (idempotent, isolated, stop)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.tsx

@@ -113,7 +113,14 @@ interface LeafViewProps {
 }
 
 function LeafView({ id, session, agent, cwd, vm, parentSplit, projectId }: LeafViewProps) {
-  const canMerge = parentSplit !== null && parentSplit.siblings > 1;
+  // A cell can be closed only when it lives inside a (binary) split: closing it
+  // collapses the parent split, keeping the *sibling*. Splits are always binary
+  // in this model (a split wraps a leaf into a 2-child container), so the kept
+  // child is simply the other index. Guarding on `siblings === 2` keeps the
+  // operation correct (merge keeps exactly one child) and avoids ever dropping
+  // the wrong terminal. The root cell (no parent split) cannot be closed.
+  const canClose = parentSplit !== null && parentSplit.siblings === 2;
+  const siblingIndex = parentSplit ? (parentSplit.index === 0 ? 1 : 0) : 0;
   const { agent: agentGateway } = useGateways();
 
   // Load the project's agents for the dropdown.
@@ -159,7 +166,10 @@ function LeafView({ id, session, agent, cwd, vm, parentSplit, projectId }: LeafV
         style={{
           position: "absolute",
           top: 2,
-          right: 2,
+          // Clear the xterm viewport scrollbar (rendered flush against the right
+          // edge, ~15px wide). Without this offset the right-most control — the
+          // close button — sits *behind* the scrollbar and is hard to click.
+          right: 16,
           zIndex: 2,
           display: "flex",
           gap: 2,
@@ -208,16 +218,14 @@ function LeafView({ id, session, agent, cwd, vm, parentSplit, projectId }: LeafV
         >
           ⬍
         </button>
-        {canMerge && (
+        {canClose && (
           <button
             type="button"
-            title="Merge: keep this cell, drop its siblings"
-            aria-label={`merge ${id}`}
-            onClick={() =>
-              void vm.merge(parentSplit.container, parentSplit.index)
-            }
+            title="Close this terminal"
+            aria-label={`close ${id}`}
+            onClick={() => void vm.merge(parentSplit.container, siblingIndex)}
           >
-            ⤬
+            ✕
           </button>
         )}
       </div>