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/grid-planning.ts

@@ -0,0 +1,107 @@
+import type { TmuxPaneInfo } from "./types"
+import {
+	DIVIDER_SIZE,
+	MAIN_PANE_RATIO,
+	MAX_GRID_SIZE,
+} from "./tmux-grid-constants"
+import { MIN_PANE_HEIGHT, MIN_PANE_WIDTH } from "./types"
+
+export interface GridCapacity {
+	cols: number
+	rows: number
+	total: number
+}
+
+export interface GridSlot {
+	row: number
+	col: number
+}
+
+export interface GridPlan {
+	cols: number
+	rows: number
+	slotWidth: number
+	slotHeight: number
+}
+
+export function calculateCapacity(
+	windowWidth: number,
+	windowHeight: number,
+): GridCapacity {
+	const availableWidth = Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
+	const cols = Math.min(
+		MAX_GRID_SIZE,
+		Math.max(
+			0,
+			Math.floor(
+				(availableWidth + DIVIDER_SIZE) / (MIN_PANE_WIDTH + DIVIDER_SIZE),
+			),
+		),
+	)
+	const rows = Math.min(
+		MAX_GRID_SIZE,
+		Math.max(
+			0,
+			Math.floor(
+				(windowHeight + DIVIDER_SIZE) / (MIN_PANE_HEIGHT + DIVIDER_SIZE),
+			),
+		),
+	)
+	return { cols, rows, total: cols * rows }
+}
+
+export function computeGridPlan(
+	windowWidth: number,
+	windowHeight: number,
+	paneCount: number,
+): GridPlan {
+	const capacity = calculateCapacity(windowWidth, windowHeight)
+	const { cols: maxCols, rows: maxRows } = capacity
+
+	if (maxCols === 0 || maxRows === 0 || paneCount === 0) {
+		return { cols: 1, rows: 1, slotWidth: 0, slotHeight: 0 }
+	}
+
+	let bestCols = 1
+	let bestRows = 1
+	let bestArea = Infinity
+
+	for (let rows = 1; rows <= maxRows; rows++) {
+		for (let cols = 1; cols <= maxCols; cols++) {
+			if (cols * rows < paneCount) continue
+			const area = cols * rows
+			if (area < bestArea || (area === bestArea && rows < bestRows)) {
+				bestCols = cols
+				bestRows = rows
+				bestArea = area
+			}
+		}
+	}
+
+	const availableWidth = Math.floor(windowWidth * (1 - MAIN_PANE_RATIO))
+	const slotWidth = Math.floor(availableWidth / bestCols)
+	const slotHeight = Math.floor(windowHeight / bestRows)
+
+	return { cols: bestCols, rows: bestRows, slotWidth, slotHeight }
+}
+
+export function mapPaneToSlot(
+	pane: TmuxPaneInfo,
+	plan: GridPlan,
+	mainPaneWidth: number,
+): GridSlot {
+	const rightAreaX = mainPaneWidth
+	const relativeX = Math.max(0, pane.left - rightAreaX)
+	const relativeY = pane.top
+
+	const col =
+		plan.slotWidth > 0
+			? Math.min(plan.cols - 1, Math.floor(relativeX / plan.slotWidth))
+			: 0
+	const row =
+		plan.slotHeight > 0
+			? Math.min(plan.rows - 1, Math.floor(relativeY / plan.slotHeight))
+			: 0
+
+	return { row, col }
+}