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>

go.mod

@@ -5,4 +5,14 @@ go 1.25.7
 require (
 	github.com/google/uuid v1.6.0
 	github.com/mattn/go-sqlite3 v1.14.37
+	google.golang.org/grpc v1.79.3
+	google.golang.org/protobuf v1.36.11
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require (
+	golang.org/x/net v0.48.0 // indirect
+	golang.org/x/sys v0.39.0 // indirect
+	golang.org/x/text v0.32.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
 )