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/agent-dispatcher.ts

@@ -10,7 +10,7 @@ import {
   setAgentModel,
   appendAgentMessages,
   updateAgentCost,
-} from './stores/agents';
+} from './stores/agents.svelte';
 
 let unlistenFn: (() => void) | null = null;