refactor(shell): simplify selectTab background-clear + autoclose lookup

- panes-fsm selectTab: replace the destructure + `void _bg` + `as TerminalTab`
  cast ceremony with a plain `{ ...tab, background: undefined }` patch.
- shell-autoclose-monitor locate(): collapse findLeafByTabId + redundant
  leaf.tabs.find re-search into one optional-chained lookup.

No behavior change. Typecheck, build, and 71 reducer/FSM tests pass.

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

Author: big-guy

src/main/panes-fsm.ts

@@ -494,12 +494,11 @@ export class PanesFSM {
       // `background` flag so its title stops rendering italic.
       const i = leaf.tabs.findIndex((t) => t.id === tabId)
       if (i !== -1 && leaf.tabs[i].background) {
-        const { background: _bg, ...rest } = leaf.tabs[i]
-        void _bg
+        const promoted: TerminalTab = { ...leaf.tabs[i], background: undefined }
         return {
           ...leaf,
           activeTabId: tabId,
-          tabs: [...leaf.tabs.slice(0, i), rest as TerminalTab, ...leaf.tabs.slice(i + 1)]
+          tabs: [...leaf.tabs.slice(0, i), promoted, ...leaf.tabs.slice(i + 1)]
         }
       }
       return { ...leaf, activeTabId: tabId }

src/main/shell-autoclose-monitor.ts

@@ -73,10 +73,8 @@ export class ShellAutoCloseMonitor {
     const panes = this.store.getSnapshot().state.terminals.panes
     for (const [wtPath, tree] of Object.entries(panes)) {
       const leaf = findLeafByTabId(tree, id)
-      if (!leaf) continue
-      const tab = leaf.tabs.find((t) => t.id === id)
-      if (!tab) continue
-      return { wtPath, tab, activeTabId: leaf.activeTabId }
+      const tab = leaf?.tabs.find((t) => t.id === id)
+      if (leaf && tab) return { wtPath, tab, activeTabId: leaf.activeTabId }
     }
     return null
   }