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.go

@@ -0,0 +1,51 @@
+// Package config provides path and endpoint configuration for the exo system.
+package config
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// Config holds all runtime configuration for exo.
+type Config struct {
+	// BasePath is the root directory for all exo data.
+	BasePath string
+
+	// DatabasePath is the path to the unified SQLite database.
+	DatabasePath string
+
+	// BlobStorePath is the root of the content-addressable blob store.
+	BlobStorePath string
+
+	// MinioEndpoint is the S3-compatible endpoint for remote blob backup.
+	MinioEndpoint string
+
+	// MinioBucket is the bucket name for blob backup.
+	MinioBucket string
+
+	// GRPCListenAddr is the address exod listens on for gRPC connections.
+	GRPCListenAddr string
+}
+
+// DefaultBasePath returns $HOME/exo.
+func DefaultBasePath() string {
+	return filepath.Join(os.Getenv("HOME"), "exo")
+}
+
+// Default returns a Config with sensible defaults rooted at $HOME/exo.
+func Default() Config {
+	base := DefaultBasePath()
+	return FromBasePath(base)
+}
+
+// FromBasePath returns a Config rooted at the given base path.
+func FromBasePath(base string) Config {
+	return Config{
+		BasePath:       base,
+		DatabasePath:   filepath.Join(base, "exo.db"),
+		BlobStorePath:  filepath.Join(base, "blobs"),
+		MinioEndpoint:  "",
+		MinioBucket:    "exo-blobs",
+		GRPCListenAddr: "localhost:9090",
+	}
+}