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)

AGENTS.md

@@ -203,7 +203,19 @@ wait for the 5-min timer. (This mechanism — and the server-side
 `setup-*.sh` scripts as of 2026-07-12; before that it silently matched
 nothing, so nothing auto-ran on any client via this path.)
 
-## 9. When in doubt
+## 9. Versioning
+
+Every commit to `main` **MUST bump the version** in the `VERSION` file at the
+repo root. The format is semver-ish: `major.minor.patch` (e.g. `0.2.3`).
+
+Rules:
+- **patch** (`0.2.2` → `0.2.3`): bugfixes, small tweaks, docs-only changes
+- **minor** (`0.2.3` → `0.3.0`): new features, new tools, visible functionality
+- **major** (`0.3.0` → `1.0.0`): breaking changes (API removal, tool retirement)
+
+The version is shown in the UI sidebar. The `v` prefix is added at build time.
+
+## 10. When in doubt
 
 Use MCP tools: `search_knowledge <query>` for narrative context,
 `get_entity <slug>` for structured data, `get_entity_knowledge <slug>` for