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.gen.yaml

@@ -0,0 +1,15 @@
+version: v2
+
+managed:
+  enabled: true
+  override:
+    - file_option: go_package_prefix
+      value: git.wntrmute.dev/kyle/exo/proto
+
+plugins:
+  - local: protoc-gen-go
+    out: ../proto
+    opt: paths=source_relative
+  - local: protoc-gen-go-grpc
+    out: ../proto
+    opt: paths=source_relative