feat(tmux-subagent): add replace action to prevent mass eviction

- Add column-based splittable calculation (getColumnCount, getColumnWidth)
- New decision tree: splittable → split, k=1 eviction → close+spawn, else → replace
- Add 'replace' action type using tmux respawn-pane (preserves layout)
- Replace oldest pane in-place instead of closing all panes when unsplittable
- Prevents scenario where all agent panes get closed leaving only 1

src/features/tmux-subagent/action-executor.ts

@@ -1,6 +1,6 @@
 import type { TmuxConfig } from "../../config/schema"
 import type { PaneAction, WindowState } from "./types"
-import { spawnTmuxPane, closeTmuxPane, enforceMainPaneWidth } from "../../shared/tmux"
+import { spawnTmuxPane, closeTmuxPane, enforceMainPaneWidth, replaceTmuxPane } from "../../shared/tmux"
 import { log } from "../../shared"
 
 export interface ActionResult {
@@ -38,6 +38,20 @@ export async function executeAction(
     return { success }
   }
 
+  if (action.type === "replace") {
+    const result = await replaceTmuxPane(
+      action.paneId,
+      action.newSessionId,
+      action.description,
+      ctx.config,
+      ctx.serverUrl
+    )
+    return {
+      success: result.success,
+      paneId: result.paneId,
+    }
+  }
+
   const result = await spawnTmuxPane(
     action.sessionId,
     action.description,
@@ -74,7 +88,7 @@ export async function executeActions(
       return { success: false, results }
     }
 
-    if (action.type === "spawn" && result.paneId) {
+    if ((action.type === "spawn" || action.type === "replace") && result.paneId) {
       spawnedPaneId = result.paneId
     }
   }