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>

artifacts/metadata.go

@@ -0,0 +1,42 @@
+package artifacts
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"git.wntrmute.dev/kyle/exo/core"
+)
+
+// StoreMetadata persists metadata key-value pairs for the given owner ID.
+func StoreMetadata(ctx context.Context, tx *sql.Tx, id string, metadata core.Metadata) error {
+	for key, value := range metadata {
+		_, err := tx.ExecContext(ctx,
+			`INSERT OR REPLACE INTO metadata (id, mkey, contents, type) VALUES (?, ?, ?, ?)`,
+			id, key, value.Contents, value.Type)
+		if err != nil {
+			return fmt.Errorf("artifacts: failed to store metadata for %s: %w", id, err)
+		}
+	}
+	return nil
+}
+
+// GetMetadata retrieves all metadata key-value pairs for the given owner ID.
+func GetMetadata(ctx context.Context, tx *sql.Tx, id string) (core.Metadata, error) {
+	metadata := core.Metadata{}
+	rows, err := tx.QueryContext(ctx,
+		`SELECT mkey, contents, type FROM metadata WHERE id=?`, id)
+	if err != nil {
+		return nil, fmt.Errorf("artifacts: failed to retrieve metadata for %s: %w", id, err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	for rows.Next() {
+		var key, contents, ctype string
+		if err := rows.Scan(&key, &contents, &ctype); err != nil {
+			return nil, fmt.Errorf("artifacts: failed to scan metadata row: %w", err)
+		}
+		metadata[key] = core.Value{Contents: contents, Type: ctype}
+	}
+	return metadata, rows.Err()
+}