feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

ui-electrobun/src/mainview/settings/ProjectSettings.svelte

@@ -2,6 +2,7 @@
   import { onMount } from 'svelte';
   import { appRpc } from '../rpc.ts';
   import { PROVIDER_CAPABILITIES, type ProviderId } from '../provider-capabilities';
+  import { setRetentionConfig } from '../agent-store.svelte.ts';
 
   const ANCHOR_SCALES = ['small', 'medium', 'large', 'full'] as const;
   type AnchorScale = typeof ANCHOR_SCALES[number];
@@ -34,6 +35,10 @@
   let selectedId = $state('p1');
   let proj = $derived(projects.find(p => p.id === selectedId)!);
 
+  // Feature 6: Retention settings
+  let sessionRetentionCount = $state(5);
+  let sessionRetentionDays = $state(30);
+
   function updateProj(patch: Partial<ProjectConfig>) {
     projects = projects.map(p => p.id === selectedId ? { ...p, ...patch } : p);
     const updated = projects.find(p => p.id === selectedId)!;
@@ -43,6 +48,17 @@
     }).catch(console.error);
   }
 
+  // Feature 6: Save retention settings
+  function updateRetention(key: string, value: number) {
+    if (key === 'count') sessionRetentionCount = value;
+    else sessionRetentionDays = value;
+    setRetentionConfig(sessionRetentionCount, sessionRetentionDays);
+    appRpc?.request['settings.set']({
+      key: key === 'count' ? 'session_retention_count' : 'session_retention_days',
+      value: String(value),
+    }).catch(console.error);
+  }
+
   onMount(async () => {
     if (!appRpc) return;
     const res = await appRpc.request['settings.getProjects']({}).catch(() => ({ projects: [] }));
@@ -52,6 +68,12 @@
       });
       if (loaded.length > 0) projects = loaded;
     }
+    // Feature 6: Load retention settings
+    try {
+      const { settings } = await appRpc.request['settings.getAll']({});
+      if (settings['session_retention_count']) sessionRetentionCount = parseInt(settings['session_retention_count'], 10) || 5;
+      if (settings['session_retention_days']) sessionRetentionDays = parseInt(settings['session_retention_days'], 10) || 30;
+    } catch { /* use defaults */ }
   });
 </script>
 
@@ -110,6 +132,23 @@
       {/each}
     </div>
 
+    <!-- Feature 6: Session retention controls -->
+    <h3 class="sh" style="margin-top: 0.625rem;">Session retention</h3>
+    <div class="slider-row">
+      <span class="lbl">Keep last</span>
+      <input type="range" min="1" max="20" step="1" value={sessionRetentionCount}
+        oninput={e => updateRetention('count', parseInt((e.target as HTMLInputElement).value, 10))}
+      />
+      <span class="slider-val">{sessionRetentionCount}</span>
+    </div>
+    <div class="slider-row">
+      <span class="lbl">Max age</span>
+      <input type="range" min="1" max="90" step="1" value={sessionRetentionDays}
+        oninput={e => updateRetention('days', parseInt((e.target as HTMLInputElement).value, 10))}
+      />
+      <span class="slider-val">{sessionRetentionDays}d</span>
+    </div>
+
     <h3 class="sh" style="margin-top: 0.625rem;">Custom context</h3>
     <textarea
       class="prompt"