fix(v2): rename rune stores to .svelte.ts to fix rune_outside_svelte error

Svelte 5 $state/$derived runes only work in .svelte and .svelte.ts
files. The stores had plain .ts extensions, causing a blank screen with
"rune_outside_svelte" runtime error. Renamed all three store files and
updated import paths across 5 consuming files.

v2/src/lib/stores/agents.svelte.ts

@@ -0,0 +1,91 @@
+// Agent tracking state — Svelte 5 runes
+// Manages agent session lifecycle and message history
+
+import type { AgentMessage } from '../adapters/sdk-messages';
+
+export type AgentStatus = 'idle' | 'starting' | 'running' | 'done' | 'error';
+
+export interface AgentSession {
+  id: string;
+  sdkSessionId?: string;
+  status: AgentStatus;
+  model?: string;
+  prompt: string;
+  messages: AgentMessage[];
+  costUsd: number;
+  inputTokens: number;
+  outputTokens: number;
+  numTurns: number;
+  durationMs: number;
+  error?: string;
+}
+
+let sessions = $state<AgentSession[]>([]);
+
+export function getAgentSessions(): AgentSession[] {
+  return sessions;
+}
+
+export function getAgentSession(id: string): AgentSession | undefined {
+  return sessions.find(s => s.id === id);
+}
+
+export function createAgentSession(id: string, prompt: string): void {
+  sessions.push({
+    id,
+    status: 'starting',
+    prompt,
+    messages: [],
+    costUsd: 0,
+    inputTokens: 0,
+    outputTokens: 0,
+    numTurns: 0,
+    durationMs: 0,
+  });
+}
+
+export function updateAgentStatus(id: string, status: AgentStatus, error?: string): void {
+  const session = sessions.find(s => s.id === id);
+  if (!session) return;
+  session.status = status;
+  if (error) session.error = error;
+}
+
+export function setAgentSdkSessionId(id: string, sdkSessionId: string): void {
+  const session = sessions.find(s => s.id === id);
+  if (session) session.sdkSessionId = sdkSessionId;
+}
+
+export function setAgentModel(id: string, model: string): void {
+  const session = sessions.find(s => s.id === id);
+  if (session) session.model = model;
+}
+
+export function appendAgentMessage(id: string, message: AgentMessage): void {
+  const session = sessions.find(s => s.id === id);
+  if (!session) return;
+  session.messages.push(message);
+}
+
+export function appendAgentMessages(id: string, messages: AgentMessage[]): void {
+  const session = sessions.find(s => s.id === id);
+  if (!session) return;
+  session.messages.push(...messages);
+}
+
+export function updateAgentCost(
+  id: string,
+  cost: { costUsd: number; inputTokens: number; outputTokens: number; numTurns: number; durationMs: number },
+): void {
+  const session = sessions.find(s => s.id === id);
+  if (!session) return;
+  session.costUsd = cost.costUsd;
+  session.inputTokens = cost.inputTokens;
+  session.outputTokens = cost.outputTokens;
+  session.numTurns = cost.numTurns;
+  session.durationMs = cost.durationMs;
+}
+
+export function removeAgentSession(id: string): void {
+  sessions = sessions.filter(s => s.id !== id);
+}