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/publisher.go

@@ -0,0 +1,70 @@
+package artifacts
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+
+	"git.wntrmute.dev/kyle/exo/core"
+)
+
+// Publisher represents a publishing entity.
+type Publisher struct {
+	ID      string
+	Name    string
+	Address string
+}
+
+// findPublisher looks up a publisher by name and address, returning its ID.
+func findPublisher(ctx context.Context, tx *sql.Tx, name, address string) (string, error) {
+	var id string
+	row := tx.QueryRowContext(ctx,
+		`SELECT id FROM publishers WHERE name=? AND address=?`, name, address)
+	if err := row.Scan(&id); err != nil {
+		return "", err
+	}
+	return id, nil
+}
+
+// Store persists a Publisher. If a publisher with the same name and address
+// already exists, it reuses that record.
+func (p *Publisher) Store(ctx context.Context, tx *sql.Tx) error {
+	if p.ID == "" {
+		id, err := findPublisher(ctx, tx, p.Name, p.Address)
+		if err == nil {
+			p.ID = id
+			return nil
+		}
+		if !errors.Is(err, sql.ErrNoRows) {
+			return fmt.Errorf("artifacts: failed to look up publisher: %w", err)
+		}
+		p.ID = core.NewUUID()
+	}
+
+	_, err := tx.ExecContext(ctx,
+		`INSERT INTO publishers (id, name, address) VALUES (?, ?, ?)`,
+		p.ID, p.Name, p.Address)
+	if err != nil {
+		return fmt.Errorf("artifacts: failed to store publisher: %w", err)
+	}
+	return nil
+}
+
+// Get retrieves a Publisher by its ID.
+func (p *Publisher) Get(ctx context.Context, tx *sql.Tx) (bool, error) {
+	if p.ID == "" {
+		return false, fmt.Errorf("artifacts: publisher missing ID: %w", core.ErrNoID)
+	}
+
+	row := tx.QueryRowContext(ctx,
+		`SELECT name, address FROM publishers WHERE id=?`, p.ID)
+	err := row.Scan(&p.Name, &p.Address)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return false, nil
+		}
+		return false, fmt.Errorf("artifacts: failed to look up publisher: %w", err)
+	}
+	return true, nil
+}