fix(tmux-panes): 60/40 layout with stacked right-column panes

First subagent gets a horizontal 60/40 split (main left, agent right).
Subsequent subagents split vertically within the right column so they
stack. rightColumnPanes tracks right-column pane order so each new
split targets the last pane in the stack.

.config/opencode/plugins/tmux-panes.ts

@@ -6,7 +6,11 @@ import { spawn } from "bun"
  *
  * When opencode spawns a background subagent, this plugin automatically opens
  * a new tmux pane showing that subagent's live TUI via `opencode attach`.
- * The pane closes when the subagent session ends.
+ *
+ * Layout:
+ *   - First subagent: horizontal 60/40 split — main pane on left, subagent on right
+ *   - Additional subagents: stacked vertically in the right column
+ *   - Panes close automatically when subagent sessions end
  *
  * Only activates when running inside a tmux session (TMUX env var is set).
  */
@@ -21,6 +25,10 @@ const plugin: Plugin = async (ctx) => {
 	const sourcePaneId = getCurrentPaneId()
 	const serverUrl = ctx.serverUrl.toString()
 
+	// Ordered list of pane IDs in the right column.
+	// Empty = no right column yet; length > 0 = right column exists.
+	const rightColumnPanes: string[] = []
+
 	return {
 		event: async ({ event }) => {
 			// Spawn a new pane when a subagent session is created
@@ -32,24 +40,45 @@ const plugin: Plugin = async (ctx) => {
 				if (sessions.has(sessionId)) return
 
 				const cmd = `opencode attach ${serverUrl} --session ${sessionId}`
-				const proc = spawn(
-					[
+
+				let args: string[]
+				if (rightColumnPanes.length === 0) {
+					// First subagent: open a horizontal split, giving the new (right)
+					// pane 40% of the window width so the main pane keeps 60%.
+					args = [
 						"tmux",
 						"split-window",
-						"-h", // horizontal split
+						"-h", // horizontal split — new pane appears on the right
+						"-p", "40", // new pane gets 40% of window width
 						"-d", // don't focus the new pane
 						"-P",
 						"-F",
-						"#{pane_id}", // print the new pane's ID
+						"#{pane_id}",
 						...(sourcePaneId ? ["-t", sourcePaneId] : []),
 						cmd,
-					],
-					{ stdout: "pipe", stderr: "pipe" },
-				)
+					]
+				} else {
+					// Additional subagents: stack vertically within the right column
+					// by splitting the last right-column pane horizontally.
+					const lastRightPane = rightColumnPanes[rightColumnPanes.length - 1]
+					args = [
+						"tmux",
+						"split-window",
+						"-v", // vertical split — new pane stacks below the target
+						"-d", // don't focus the new pane
+						"-P",
+						"-F",
+						"#{pane_id}",
+						"-t", lastRightPane,
+						cmd,
+					]
+				}
 
+				const proc = spawn(args, { stdout: "pipe", stderr: "pipe" })
 				const paneId = (await new Response(proc.stdout).text()).trim()
 				if ((await proc.exited) === 0 && paneId) {
 					sessions.set(sessionId, paneId)
+					rightColumnPanes.push(paneId)
 				}
 			}
 
@@ -63,6 +92,8 @@ const plugin: Plugin = async (ctx) => {
 						stderr: "ignore",
 					})
 					sessions.delete(info.id)
+					const idx = rightColumnPanes.indexOf(paneId)
+					if (idx !== -1) rightColumnPanes.splice(idx, 1)
 				}
 			}
 		},