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/manager.ts

@@ -166,11 +166,11 @@ export class TmuxSessionManager {
         canSpawn: decision.canSpawn,
         reason: decision.reason,
         actionCount: decision.actions.length,
-        actions: decision.actions.map((a) =>
-          a.type === "close"
-            ? { type: "close", paneId: a.paneId }
-            : { type: "spawn", sessionId: a.sessionId }
-        ),
+        actions: decision.actions.map((a) => {
+          if (a.type === "close") return { type: "close", paneId: a.paneId }
+          if (a.type === "replace") return { type: "replace", paneId: a.paneId, newSessionId: a.newSessionId }
+          return { type: "spawn", sessionId: a.sessionId }
+        }),
       })
 
       if (!decision.canSpawn) {
@@ -190,6 +190,13 @@ export class TmuxSessionManager {
             sessionId: action.sessionId,
           })
         }
+        if (action.type === "replace" && actionResult.success) {
+          this.sessions.delete(action.oldSessionId)
+          log("[tmux-session-manager] removed replaced session from cache", {
+            oldSessionId: action.oldSessionId,
+            newSessionId: action.newSessionId,
+          })
+        }
       }
 
       if (result.success && result.spawnedPaneId) {