refactor(tmux-subagent): split manager and decision-engine into focused modules

Extract session lifecycle, polling, grid planning, and event handling:
- polling.ts: session polling controller with stability detection
- event-handlers.ts: session created/deleted handlers
- grid-planning.ts, spawn-action-decider.ts, spawn-target-finder.ts
- session-status-parser.ts, session-message-count.ts
- cleanup.ts, polling-constants.ts, tmux-grid-constants.ts

src/features/tmux-subagent/pane-split-availability.ts

@@ -0,0 +1,60 @@
+import type { SplitDirection, TmuxPaneInfo } from "./types"
+import {
+	DIVIDER_SIZE,
+	MAX_COLS,
+	MAX_ROWS,
+	MIN_SPLIT_HEIGHT,
+	MIN_SPLIT_WIDTH,
+} from "./tmux-grid-constants"
+
+export function getColumnCount(paneCount: number): number {
+	if (paneCount <= 0) return 1
+	return Math.min(MAX_COLS, Math.max(1, Math.ceil(paneCount / MAX_ROWS)))
+}
+
+export function getColumnWidth(agentAreaWidth: number, paneCount: number): number {
+	const cols = getColumnCount(paneCount)
+	const dividersWidth = (cols - 1) * DIVIDER_SIZE
+	return Math.floor((agentAreaWidth - dividersWidth) / cols)
+}
+
+export function isSplittableAtCount(
+	agentAreaWidth: number,
+	paneCount: number,
+): boolean {
+	const columnWidth = getColumnWidth(agentAreaWidth, paneCount)
+	return columnWidth >= MIN_SPLIT_WIDTH
+}
+
+export function findMinimalEvictions(
+	agentAreaWidth: number,
+	currentCount: number,
+): number | null {
+	for (let k = 1; k <= currentCount; k++) {
+		if (isSplittableAtCount(agentAreaWidth, currentCount - k)) {
+			return k
+		}
+	}
+	return null
+}
+
+export function canSplitPane(pane: TmuxPaneInfo, direction: SplitDirection): boolean {
+	if (direction === "-h") {
+		return pane.width >= MIN_SPLIT_WIDTH
+	}
+	return pane.height >= MIN_SPLIT_HEIGHT
+}
+
+export function canSplitPaneAnyDirection(pane: TmuxPaneInfo): boolean {
+	return pane.width >= MIN_SPLIT_WIDTH || pane.height >= MIN_SPLIT_HEIGHT
+}
+
+export function getBestSplitDirection(pane: TmuxPaneInfo): SplitDirection | null {
+	const canH = pane.width >= MIN_SPLIT_WIDTH
+	const canV = pane.height >= MIN_SPLIT_HEIGHT
+
+	if (!canH && !canV) return null
+	if (canH && !canV) return "-h"
+	if (!canH && canV) return "-v"
+	return pane.width >= pane.height ? "-h" : "-v"
+}