add version display in UI sidebar + version bump rules

- VERSION file at repo root (0.3.0)
- vite.config.ts reads VERSION at build time, injects __OIKOS_VERSION__
- App.svelte shows version in sidebar tooltip + subtle text below logo
- AGENTS.md §9: every commit to main MUST bump VERSION
  (patch=bugfix, minor=new features, major=breaking changes)

web/src/App.svelte

@@ -19,6 +19,7 @@
   import { Button } from '$lib/components/ui/button'
   import { Badge } from '$lib/components/ui/badge'
   import { Separator } from '$lib/components/ui/separator'
+  import { VERSION } from '$lib/version'
   import { Toaster } from '$lib/components/ui/sonner'
   import PlusIcon from '@lucide/svelte/icons/plus'
   import LayoutDashboardIcon from '@lucide/svelte/icons/layout-dashboard'
@@ -99,7 +100,7 @@
           <Sidebar.MenuButton
             class="data-[slot=sidebar-menu-button]:!p-1.5"
             onclick={() => navigate('overview')}
-            tooltipContent="Oikos"
+            tooltipContent={`Oikos ${VERSION}`}
           >
             {#snippet child({ props })}
               <button {...props}>
@@ -112,6 +113,9 @@
           </Sidebar.MenuButton>
         </Sidebar.MenuItem>
       </Sidebar.Menu>
+      <div class="group-data-[collapsible=icon]:hidden px-2.5 pb-1">
+        <span class="text-[11px] text-muted-foreground select-none">{VERSION}</span>
+      </div>
       <Sidebar.Menu>
         <Sidebar.MenuItem>
           <Sidebar.MenuButton