feat: add Obsidian vault projection with Longhorn storage

- Create PVC (10Gi) for vault on Longhorn persistent storage
- Update deployment to mount vault volume instead of emptyDir
- Add POST /memory/vault endpoint to generate markdown vault
- Generates one .md file per L1 memory (YAML frontmatter + content)
- Index.md for each project from L2 synthesis
- Auto-update on ingest completion via vault endpoint

Vault structure:
  /data/vault/
    poimen/
      index.md              (L2 synthesis)
      architecture.md       (L1 memory for 'architecture-decisions')
      infra-root-causes.md  (L1 memory for 'infra-root-causes')

Ready for human reading in Obsidian app or programmatic access.

k8s/app/deployment.yaml

@@ -87,7 +87,8 @@ spec:
               mountPath: /data
       volumes:
         - name: data
-          emptyDir: {}
+          persistentVolumeClaim:
+            claimName: poimen-memory-vault
       # Tolerate control-plane nodes
       tolerations:
         - key: node-role.kubernetes.io/control-plane