Add Phase 1 foundation: Go module, core types, DB infrastructure, config

Establish the project foundation with three packages:
- core: shared types (Header, Metadata, Value, ObjectType, UUID generation)
- db: SQLite migration framework, connection management (WAL, FK, busy
  timeout), transaction helpers (StartTX/EndTX), time conversion
- config: runtime configuration (DB path, blob store, Minio, gRPC addr)

Includes initial schema migration (001_initial.sql) with 13 tables covering
shared infrastructure, bibliographic data, and artifact repository. Full test
coverage for all packages, strict linting (.golangci.yaml), and Makefile.

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

config/config_test.go

@@ -0,0 +1,39 @@
+package config
+
+import (
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestDefault(t *testing.T) {
+	cfg := Default()
+	if cfg.BasePath == "" {
+		t.Fatal("BasePath is empty")
+	}
+	if !strings.HasSuffix(cfg.BasePath, "exo") {
+		t.Fatalf("BasePath should end with 'exo', got %q", cfg.BasePath)
+	}
+	if cfg.DatabasePath == "" {
+		t.Fatal("DatabasePath is empty")
+	}
+	if cfg.BlobStorePath == "" {
+		t.Fatal("BlobStorePath is empty")
+	}
+	if cfg.GRPCListenAddr == "" {
+		t.Fatal("GRPCListenAddr is empty")
+	}
+}
+
+func TestFromBasePath(t *testing.T) {
+	cfg := FromBasePath("/tmp/testexo")
+	if cfg.BasePath != "/tmp/testexo" {
+		t.Fatalf("expected BasePath '/tmp/testexo', got %q", cfg.BasePath)
+	}
+	if cfg.DatabasePath != filepath.Join("/tmp/testexo", "exo.db") {
+		t.Fatalf("unexpected DatabasePath: %q", cfg.DatabasePath)
+	}
+	if cfg.BlobStorePath != filepath.Join("/tmp/testexo", "blobs") {
+		t.Fatalf("unexpected BlobStorePath: %q", cfg.BlobStorePath)
+	}
+}