Add Phase 2 artifact repository: types, blob store, gRPC service

Build the complete artifact pillar with five packages:
- artifacts: Artifact, Snapshot, Citation, Publisher types with Get/Store
  DB methods, tag/category management, metadata ops, YAML import
- blob: content-addressable store (SHA256, hierarchical dir layout)
- proto: protobuf definitions (common.proto, artifacts.proto) with buf
  linting and code generation
- server: gRPC ArtifactService implementation (create/get artifacts,
  store/retrieve blobs, manage tags/categories, search by tag)

All FK insertion ordering is correct (parent rows before children).
Full test coverage across artifacts, blob, and server packages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

proto/buf.yaml

@@ -0,0 +1,12 @@
+version: v2
+
+modules:
+  - path: .
+
+lint:
+  use:
+    - STANDARD
+
+breaking:
+  use:
+    - FILE