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/oldest-agent-pane.ts

@@ -0,0 +1,37 @@
+import type { TmuxPaneInfo } from "./types"
+
+export interface SessionMapping {
+	sessionId: string
+	paneId: string
+	createdAt: Date
+}
+
+export function findOldestAgentPane(
+	agentPanes: TmuxPaneInfo[],
+	sessionMappings: SessionMapping[],
+): TmuxPaneInfo | null {
+	if (agentPanes.length === 0) return null
+
+	const paneIdToAge = new Map<string, Date>()
+	for (const mapping of sessionMappings) {
+		paneIdToAge.set(mapping.paneId, mapping.createdAt)
+	}
+
+	const panesWithAge = agentPanes
+		.map((pane) => ({ pane, age: paneIdToAge.get(pane.paneId) }))
+		.filter(
+			(item): item is { pane: TmuxPaneInfo; age: Date } => item.age !== undefined,
+		)
+		.sort((a, b) => a.age.getTime() - b.age.getTime())
+
+	if (panesWithAge.length > 0) {
+		return panesWithAge[0].pane
+	}
+
+	return agentPanes.reduce((oldest, pane) => {
+		if (pane.top < oldest.top || (pane.top === oldest.top && pane.left < oldest.left)) {
+			return pane
+		}
+		return oldest
+	})
+}