feat(db): add SQLite schema, Store CRUD, ULID generation

Foundation layer: entities table with card support, entity_tags join
table, WAL mode, busy_timeout, full CRUD operations including
promote/demote lifecycle and soft/hard delete. 33 tests passing.

internal/db/db.go

@@ -0,0 +1,102 @@
+package db
+
+import (
+	"database/sql"
+	"errors"
+	"os"
+	"path/filepath"
+
+	_ "modernc.org/sqlite"
+)
+
+var (
+	ErrNotFound        = errors.New("not_found")
+	ErrAlreadyPromoted = errors.New("invalid_promote")
+	ErrAlreadyFluid    = errors.New("invalid_demote")
+)
+
+type Store struct {
+	db *sql.DB
+}
+
+func Open(path string) (*Store, error) {
+	db, err := sql.Open("sqlite", path)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, pragma := range []string{
+		"PRAGMA journal_mode = WAL",
+		"PRAGMA foreign_keys = ON",
+		"PRAGMA busy_timeout = 5000",
+	} {
+		if _, err := db.Exec(pragma); err != nil {
+			db.Close()
+			return nil, err
+		}
+	}
+
+	s := &Store{db: db}
+	if err := s.migrate(); err != nil {
+		db.Close()
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *Store) Close() error {
+	return s.db.Close()
+}
+
+func (s *Store) migrate() error {
+	_, err := s.db.Exec(`
+		CREATE TABLE IF NOT EXISTS entities (
+			id          TEXT PRIMARY KEY,
+			created_at  TEXT NOT NULL,
+			modified_at TEXT NOT NULL,
+			body        TEXT NOT NULL,
+			glyph       TEXT NOT NULL
+				CHECK (glyph IN ('todo', 'event', 'note')),
+			time_anchor TEXT,
+			completed_at TEXT,
+			pinned      INTEGER NOT NULL DEFAULT 0,
+			deleted_at  TEXT,
+			card_type   TEXT
+				CHECK (card_type IN ('snippet', 'template', 'checklist', 'decision', 'link')
+				       OR card_type IS NULL),
+			card_data   TEXT,
+			use_count   INTEGER NOT NULL DEFAULT 0,
+			last_used_at TEXT
+		);
+
+		CREATE TABLE IF NOT EXISTS entity_tags (
+			entity_id TEXT NOT NULL REFERENCES entities(id) ON DELETE CASCADE,
+			tag       TEXT NOT NULL,
+			PRIMARY KEY (entity_id, tag)
+		);
+
+		CREATE INDEX IF NOT EXISTS idx_entities_created
+			ON entities(created_at DESC) WHERE deleted_at IS NULL;
+		CREATE INDEX IF NOT EXISTS idx_entities_card_use
+			ON entities(use_count DESC)
+			WHERE card_type IS NOT NULL AND deleted_at IS NULL;
+		CREATE INDEX IF NOT EXISTS idx_entity_tags_tag
+			ON entity_tags(tag);
+	`)
+	return err
+}
+
+func DefaultPath() (string, error) {
+	if env := os.Getenv("NIB_DB"); env != "" {
+		return env, nil
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	dir := filepath.Join(home, ".nib")
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, "nib.db"), nil
+}