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/spawn-action-decider.ts

@@ -0,0 +1,135 @@
+import type {
+	CapacityConfig,
+	PaneAction,
+	SpawnDecision,
+	TmuxPaneInfo,
+	WindowState,
+} from "./types"
+import { MAIN_PANE_RATIO } from "./tmux-grid-constants"
+import {
+	canSplitPane,
+	findMinimalEvictions,
+	isSplittableAtCount,
+} from "./pane-split-availability"
+import { findSpawnTarget } from "./spawn-target-finder"
+import { findOldestAgentPane, type SessionMapping } from "./oldest-agent-pane"
+import { MIN_PANE_WIDTH } from "./types"
+
+export function decideSpawnActions(
+	state: WindowState,
+	sessionId: string,
+	description: string,
+	_config: CapacityConfig,
+	sessionMappings: SessionMapping[],
+): SpawnDecision {
+	if (!state.mainPane) {
+		return { canSpawn: false, actions: [], reason: "no main pane found" }
+	}
+
+	const agentAreaWidth = Math.floor(state.windowWidth * (1 - MAIN_PANE_RATIO))
+	const currentCount = state.agentPanes.length
+
+	if (agentAreaWidth < MIN_PANE_WIDTH) {
+		return {
+			canSpawn: false,
+			actions: [],
+			reason: `window too small for agent panes: ${state.windowWidth}x${state.windowHeight}`,
+		}
+	}
+
+	const oldestPane = findOldestAgentPane(state.agentPanes, sessionMappings)
+	const oldestMapping = oldestPane
+		? sessionMappings.find((m) => m.paneId === oldestPane.paneId) ?? null
+		: null
+
+	if (currentCount === 0) {
+		const virtualMainPane: TmuxPaneInfo = { ...state.mainPane, width: state.windowWidth }
+		if (canSplitPane(virtualMainPane, "-h")) {
+			return {
+				canSpawn: true,
+				actions: [
+					{
+						type: "spawn",
+						sessionId,
+						description,
+						targetPaneId: state.mainPane.paneId,
+						splitDirection: "-h",
+					},
+				],
+			}
+		}
+		return { canSpawn: false, actions: [], reason: "mainPane too small to split" }
+	}
+
+	if (isSplittableAtCount(agentAreaWidth, currentCount)) {
+		const spawnTarget = findSpawnTarget(state)
+		if (spawnTarget) {
+			return {
+				canSpawn: true,
+				actions: [
+					{
+						type: "spawn",
+						sessionId,
+						description,
+						targetPaneId: spawnTarget.targetPaneId,
+						splitDirection: spawnTarget.splitDirection,
+					},
+				],
+			}
+		}
+	}
+
+	const minEvictions = findMinimalEvictions(agentAreaWidth, currentCount)
+	if (minEvictions === 1 && oldestPane) {
+		return {
+			canSpawn: true,
+			actions: [
+				{
+					type: "close",
+					paneId: oldestPane.paneId,
+					sessionId: oldestMapping?.sessionId || "",
+				},
+				{
+					type: "spawn",
+					sessionId,
+					description,
+					targetPaneId: state.mainPane.paneId,
+					splitDirection: "-h",
+				},
+			],
+			reason: "closed 1 pane to make room for split",
+		}
+	}
+
+	if (oldestPane) {
+		return {
+			canSpawn: true,
+			actions: [
+				{
+					type: "replace",
+					paneId: oldestPane.paneId,
+					oldSessionId: oldestMapping?.sessionId || "",
+					newSessionId: sessionId,
+					description,
+				},
+			],
+			reason: "replaced oldest pane (no split possible)",
+		}
+	}
+
+	return { canSpawn: false, actions: [], reason: "no pane available to replace" }
+}
+
+export function decideCloseAction(
+	state: WindowState,
+	sessionId: string,
+	sessionMappings: SessionMapping[],
+): PaneAction | null {
+	const mapping = sessionMappings.find((m) => m.sessionId === sessionId)
+	if (!mapping) return null
+
+	const paneExists = state.agentPanes.some((pane) => pane.paneId === mapping.paneId)
+	if (!paneExists) return null
+
+	return { type: "close", paneId: mapping.paneId, sessionId }
+}