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/api/entities.go

@@ -304,6 +304,50 @@ func demoteEntity(store *db.Store) http.HandlerFunc {
 	}
 }
 
+type AbsorbRequest struct {
+	SourceID string `json:"source_id"`
+}
+
+func absorbEntity(store *db.Store) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		id := chi.URLParam(r, "id")
+
+		var req AbsorbRequest
+		if !decodeJSON(w, r, &req) {
+			return
+		}
+
+		if req.SourceID == "" {
+			writeError(w, http.StatusBadRequest, "invalid_input", "source_id is required")
+			return
+		}
+		if req.SourceID == id {
+			writeError(w, http.StatusBadRequest, "invalid_input", "target and source must be different entities")
+			return
+		}
+
+		if err := store.Absorb(id, req.SourceID); err != nil {
+			if err == db.ErrNotFound {
+				writeError(w, http.StatusNotFound, "not_found", "target or source entity not found")
+				return
+			}
+			if err == db.ErrTargetCrystallized {
+				writeError(w, http.StatusBadRequest, "invalid_absorb", "target is crystallized — demote first")
+				return
+			}
+			writeError(w, http.StatusInternalServerError, "internal", err.Error())
+			return
+		}
+
+		e, err := store.Get(id)
+		if err != nil {
+			writeError(w, http.StatusInternalServerError, "internal", err.Error())
+			return
+		}
+		writeJSON(w, http.StatusOK, entityToResponse(e))
+	}
+}
+
 func useEntity(store *db.Store) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		id := chi.URLParam(r, "id")