fix: align split targeting with configured pane width

Use the configured agent pane width consistently in split target selection and avoid close+spawn churn by replacing the oldest pane when eviction is required.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/tmux-subagent/spawn-action-decider.ts

@@ -62,7 +62,7 @@ export function decideSpawnActions(
 	}
 
 	if (isSplittableAtCount(agentAreaWidth, currentCount, minPaneWidth)) {
-		const spawnTarget = findSpawnTarget(state)
+		const spawnTarget = findSpawnTarget(state, minPaneWidth)
 		if (spawnTarget) {
 			return {
 				canSpawn: true,
@@ -85,19 +85,14 @@ export function decideSpawnActions(
 			canSpawn: true,
 			actions: [
 				{
-					type: "close",
+					type: "replace",
 					paneId: oldestPane.paneId,
-					sessionId: oldestMapping?.sessionId || "",
-				},
-				{
-					type: "spawn",
-					sessionId,
+					oldSessionId: oldestMapping?.sessionId || "",
+					newSessionId: sessionId,
 					description,
-					targetPaneId: state.mainPane.paneId,
-					splitDirection: "-h",
 				},
 			],
-			reason: "closed 1 pane to make room for split",
+			reason: "replaced oldest pane to avoid split churn",
 		}
 	}