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/router.go

@@ -31,6 +31,7 @@ func NewRouter(store *db.Store, devMode bool, webFS ...fs.FS) chi.Router {
 		r.Post("/entities/{id}/promote", promoteEntity(store))
 		r.Post("/entities/{id}/demote", demoteEntity(store))
 		r.Post("/entities/{id}/use", useEntity(store))
+		r.Post("/entities/{id}/absorb", absorbEntity(store))
 		r.Get("/tags", listTags(store))
 	})