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/App.svelte

@@ -2,7 +2,7 @@
   import { onMount, onDestroy } from 'svelte';
   import SessionList from './lib/components/Sidebar/SessionList.svelte';
   import TilingGrid from './lib/components/Layout/TilingGrid.svelte';
-  import { addPane, focusPaneByIndex, getPanes } from './lib/stores/layout';
+  import { addPane, focusPaneByIndex, getPanes } from './lib/stores/layout.svelte';
   import { startAgentDispatcher, stopAgentDispatcher } from './lib/agent-dispatcher';
 
   function newTerminal() {

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;
 

v2/src/lib/components/Agent/AgentPane.svelte

@@ -6,7 +6,7 @@
     createAgentSession,
     removeAgentSession,
     type AgentSession,
-  } from '../../stores/agents';
+  } from '../../stores/agents.svelte';
   import type {
     AgentMessage,
     TextContent,
@@ -49,7 +49,7 @@
     const ready = await isAgentReady();
     if (!ready) {
       createAgentSession(sessionId, text);
-      const { updateAgentStatus } = await import('../../stores/agents');
+      const { updateAgentStatus } = await import('../../stores/agents.svelte');
       updateAgentStatus(sessionId, 'error', 'Sidecar not ready — agent features unavailable');
       return;
     }

v2/src/lib/components/Layout/TilingGrid.svelte

@@ -8,7 +8,7 @@
     getPaneGridArea,
     focusPane,
     removePane,
-  } from '../../stores/layout';
+  } from '../../stores/layout.svelte';
 
   let gridTemplate = $derived(getGridTemplate());
   let panes = $derived(getPanes());

v2/src/lib/components/Sidebar/SessionList.svelte

@@ -7,7 +7,7 @@
     getActivePreset,
     setPreset,
     type LayoutPreset,
-  } from '../../stores/layout';
+  } from '../../stores/layout.svelte';
 
   let panes = $derived(getPanes());
   let preset = $derived(getActivePreset());