fix(tmux-subagent): enable 2D grid layout with divider-aware calculations

- Account for tmux pane dividers (1 char) in all size calculations
- Reduce MIN_PANE_WIDTH from 53 to 52 to fit 2 columns in standard terminals
- Fix enforceMainPaneWidth to use (windowWidth - divider) / 2
- Add virtual mainPane handling for close-spawn eviction loop
- Add comprehensive decision-engine tests (23 test cases)

src/shared/tmux/tmux-utils.ts

@@ -125,7 +125,6 @@ export async function spawnTmuxPane(
     "-P",
     "-F",
     "#{pane_id}",
-    "-l", String(config.agent_pane_min_width),
     ...(targetPaneId ? ["-t", targetPaneId] : []),
     opencodeCmd,
   ]
@@ -185,14 +184,36 @@ export async function applyLayout(
   layout: TmuxLayout,
   mainPaneSize: number
 ): Promise<void> {
-  spawn([tmux, "select-layout", layout], { stdout: "ignore", stderr: "ignore" })
+  const layoutProc = spawn([tmux, "select-layout", layout], { stdout: "ignore", stderr: "ignore" })
+  await layoutProc.exited
 
   if (layout.startsWith("main-")) {
     const dimension =
       layout === "main-horizontal" ? "main-pane-height" : "main-pane-width"
-    spawn([tmux, "set-window-option", dimension, `${mainPaneSize}%`], {
+    const sizeProc = spawn([tmux, "set-window-option", dimension, `${mainPaneSize}%`], {
       stdout: "ignore",
       stderr: "ignore",
     })
+    await sizeProc.exited
   }
 }
+
+export async function enforceMainPaneWidth(
+  mainPaneId: string,
+  windowWidth: number
+): Promise<void> {
+  const { log } = await import("../logger")
+  const tmux = await getTmuxPath()
+  if (!tmux) return
+
+  const DIVIDER_WIDTH = 1
+  const mainWidth = Math.floor((windowWidth - DIVIDER_WIDTH) / 2)
+  
+  const proc = spawn([tmux, "resize-pane", "-t", mainPaneId, "-x", String(mainWidth)], {
+    stdout: "ignore",
+    stderr: "ignore",
+  })
+  await proc.exited
+  
+  log("[enforceMainPaneWidth] main pane resized", { mainPaneId, mainWidth, windowWidth })
+}