feat: add absorb command — merge source entity into target

DB: Absorb() merges body (newline-separated), unions tags, demotes
crystallized sources, soft-deletes source. Rejects crystallized targets.

API: POST /api/entities/:id/absorb { source_id }

CLI: nib absorb <target> <source> with prefix ID resolution

Web: absorb button on fluid entities, 'a' keyboard shortcut,
source picker modal

internal/db/entities.go

@@ -405,6 +405,63 @@ func (s *Store) SoftDelete(id string) (DeleteResult, error) {
 	return DeletedSoft, err
 }
 
+func (s *Store) Absorb(targetID, sourceID string) error {
+	target, err := s.Get(targetID)
+	if err != nil {
+		return err
+	}
+	source, err := s.Get(sourceID)
+	if err != nil {
+		return err
+	}
+
+	if target.CardType != nil {
+		return ErrTargetCrystallized
+	}
+
+	tx, err := s.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	now := time.Now().UTC().Format(time.RFC3339)
+	merged := target.Body + "\n" + source.Body
+
+	if _, err := tx.Exec("UPDATE entities SET body = ?, modified_at = ? WHERE id = ?",
+		merged, now, targetID); err != nil {
+		return err
+	}
+
+	seen := map[string]bool{}
+	for _, t := range target.Tags {
+		seen[t] = true
+	}
+	for _, t := range source.Tags {
+		if !seen[t] {
+			if _, err := tx.Exec("INSERT OR IGNORE INTO entity_tags (entity_id, tag) VALUES (?, ?)",
+				targetID, t); err != nil {
+				return err
+			}
+		}
+	}
+
+	if source.CardType != nil {
+		if _, err := tx.Exec(`UPDATE entities SET card_type = NULL, card_data = NULL,
+			use_count = 0, last_used_at = NULL, modified_at = ? WHERE id = ?`,
+			now, sourceID); err != nil {
+			return err
+		}
+	}
+
+	if _, err := tx.Exec("UPDATE entities SET deleted_at = ? WHERE id = ?",
+		now, sourceID); err != nil {
+		return err
+	}
+
+	return tx.Commit()
+}
+
 func (s *Store) IncrementUse(id string) error {
 	res, err := s.db.Exec(`
 		UPDATE entities SET use_count = use_count + 1, last_used_at = ?