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/kustomization.yaml

@@ -2,6 +2,7 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 namespace: poimen
 resources:
+  - vault-pvc.yaml
   - deployment.yaml
   - service.yaml
   # Secret managed separately (SealedSecret in homelab)